work shift duration: a review comparing eight hour and 12 hour shift systems

REVIEW

Work shift duration: a review comparing eighthour and 12 hour shift systems

Lawrence Smith, Simon Folkard, Phil Tucker, Ian Macdonald

AbstractObjectives—Shiftwork is now a major fea-ture of working life across a broad range ofindustries. The features of the shift sys-tems operated can impact on the wellbe-ing, performance, and sleep ofshiftworkers. This paper reviews the cur-rent state of knowledge on one majorcharacteristic of shift rotas—namely, shiftduration. Evidence comparing the relativeeVects of eight hour and 12 hour shifts onfatigue and job performance, safety, sleep,and physical and psychological health areconsidered. At the organisational level,factors such as the mode of system imple-mentation, attitudes towards shift rotas,sickness absence and turnover, overtime,and moonlighting are discussed.Methods—Manual and electronic searchesof the shiftwork research literature wereconducted to obtain information on com-parisons between eight hour and 12 hourshifts.Results—The research findings arelargely equivocal. The bulk of the evidencesuggests few diVerences between eight and12 hour shifts in the way they aVectpeople. There may even be advantages to12 hour shifts in terms of lower stress lev-els, better physical and psychologicalwellbeing, improved durations and qualityof oV duty sleep as well as improvementsin family relations. On the negative side,the main concerns are fatigue and safety.It is noted that a 12 hour shift does notequate with being active for only 12 hours.Conclusions—There can be considerableextension of the person’s time awakeeither side of the shift. However, theeVects of longer term exposure to ex-tended work days have been relativelyuncharted in any systematic way. Longitu-dinal comparative research into thechronic impact of the compressed workingweek is needed.

(Occup Environ Med 1998;55:217–229)

Keywords: shift duration; health; sleep; 12 hour shifts

In response to economic, technological, andsocial pressures a 24 hour organisation of the

workplace is now common. This requires theworkforce to accept and become adapted tomany diVerent forms of shiftwork schedule.However, the adverse eVects of shiftworkingare well chronicled.1–3 They include biologicaldisruption to physiological processes, includ-ing the sleep-wake cycle4; the impairment ofphysical health and psychological wellbeing3 5;problems with alertness, performance, andsafety,6 7 and lastly, interference with social anddomestic life.8 9 The extent to which shiftworkaVects the individual person depends largelyon the job being done, characteristics of theindividual, the organisational and social envi-ronments, and features of the shift system.Thispotential impairment to functioning existsbecause daily rhythms in human physiology,hormone concentrations, biochemistry, andbehaviour have become entrained to the mostreliable and predictable cyclic changes in thephysical and social environments. Under shift-working conditions, and especially when re-quired to work at night, the components of thissystem of circadian rhythms are regularlydisrupted as a result of having to alter theactivity-rest cycle.Many thousands of shiftwork schedules are

known to be in operation world wide butconsideration of shift duration highlights twocommon categories of shift systems, each withnumerous permutations, designed to provide24 hour cover—namely, systems comprisingthree×eight hour shifts or two×12 hour shifts.Twelve hour rotas represent a compressedworking week which has been defined as: “Anysystem of fixed working hours more than eighthours in duration which results in a workingweek of less than five full days of work aweek”.10 Conversion to a compressed workingweek is often considered because of theperceived potential benefits for production andmorale, reduction of sickness absence,11 andthe convenience of having extra time oV work.12

A recent Bulletin of European Studies of Time13

considers many of the issues surrounding thecompressed working week. Schedules involvingshifts greater than eight hours and fewer thanfive full days at work (including 12 hour sched-ules) are discussed somewhat interchangeably.That is, a rather broad brush approach is gen-erally taken. This paper also intends to take abroad view as it reviews the shiftwork literaturethat compares the impact of eight and 12 hour

Occup Environ Med 1998;55:217–229 217

Shiftwork and SafetyResearch Group,School of Psychology,University of Leeds,Leeds, UKL Smith

MRC Body Rhythmand Shiftwork Centre,Department ofPsychology, UniversityCollege of Wales,Swansea, SingletonPark, Swansea, UKS FolkardP TuckerI Macdonald

Correspondence to:Dr Lawrence Smith,Shiftwork and SafetyResearch Group, School ofPsychology, University ofLeeds, Leeds LS2 9JT, UK.

Accepted 25 November 1997

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shift rotas on the sleep, health, performance,and safety of the people concerned. In doing sowe cite information ranging from anecdotalevidence to field based research. The overrid-ing objective is to provide the interested readerwith a comprehensive overview of the debateon shift durations.This review aims to provide evidence from a

wide range of information, much of which isinaccessible to many interested in the area ofeVects of working time. The sections whichfollow consider information relating to theimplications of eight and 12 hour shifts for: (a)fatigue and performance in work; (b) safety; (c)sleep, physical health, and psychosocial wellbe-ing; (d) system implementation, shiftworkerattitudes, preferences, and morale; (e) absen-teeism and turnover; and (f) overtime andmoonlighting.

MethodThe primary sources of information wereresearch project reports and theoretical orresearch based peer reviewed journal papers.These were found by extensive manual andelectronic literature searches conducted on: theInternet; the United Kingdom Health andSafety Executive CD-ROM database accessedat the Health and Safety Executive Library,Broad Lane, SheYeld, United Kingdom;MedLine CD-ROM database accessed on theinternet; Psych-Lit CD-ROM database ac-cessed on the Internet; and the social sciencescitation index. Also, authors of reports andarticles not widely available were contacted.13a

ResultsMuch of the research and commentary com-paring eight and 12 hour systems originates(although not exclusively) in two major sectorsof industry. One source has been the healthservice where the best type of shift system topromote patient care and staV satisfaction hasbeen the subject of considerable debate in theliterature related to healthcare shiftwork, espe-cially in the United States. The other sourcehas been the nuclear industry where safeoperations are the prime concern and the pos-sible eVects induced by fatigue of longer work-shifts need to be carefully examined.Twelve hour systems are popular with

shiftworkers because they compress the work-ing week and oVer greater opportunity for timeaway from work. As we shall see, after theintroduction of 12 hour shifts some studieshave reported benefits in positive attitudesabout travel to work and time oV duty,improved staV morale, and reduced sicknessabsence.14 15 However, unfavourable outcomeshave also been reported after the introductionof 12 hour shifts. For example, reduction inquality of patient care and dislike of the shifthave been cited as major problems inhospitals.16 17

The major arguments against 12 hour shiftstend to revolve around the concern that theypromote fatigue and compromise alertness andperformance, thereby reducing operationaleYciency or safety.18 Consequently, the work-time research literature cautions against longer

work shifts, especially 12 hour night shifts,where public safety is a main consideration.However, the research evidence on compressedworking weeks tends to be less supportive ofthis position. For example in a comprehensivereview and meta-analysis of empirical studiesinto the eVects of compressed working weeks, ithas been concluded that, if anything, theimpact of such scheduling is beneficial.19 It wasshown that compressed schedules broughtmoderate decreases in sickness absence, smallincreases in productivity, large increases inschedule and job satisfaction, but also moder-ate increases in fatigue.From just these introductory examples it can

immediately be seen that the evidence fromresearch comparing eight hour and 12 hourshifts can be complex and possibly contradic-tory. The following sections will consider theevidence for diVerences between eight hourand 12 hour systems and try to drawconclusions about their implications for severalaspects of shiftworkers’ lives and employingorganisations.

FATIGUE AND JOB PERFORMANCE

Earlier comparisons of job performance werenot favourable to the 12 hour systems. TheBritish Industrial Fatigue Research Boardreported apparent changes in productivity afterthe reduction of shift times from 12 hours toeight hours at a steel mill between 1910 and1914.20 Production details were obtained fortwo years before and two years after the changeto the three × eight system at the steel works. Agradual decrease in output over the 12 hourperiod was reported. After no immediatechange in output in the first two months of theeight hour rota, there followed an increase inproductivity which reached a peak 13 monthsafter the reduction in shift hours before startinga gradual decline. Vernon20 concluded that thisrepresented a significant increase (about 18%)in output which could not be masked by thesubsequent decline.Osborne21 examined diVerences in produc-

tion output and fatigue at a munitions factoryrelative to eight hour and 12 hour shifts. Forwork which was self paced the equivalentamount of work completed in 100 minutes ona 12 hour system was completed in 80.5minutes on the eight hour system. On all meas-ures there was an apparent advantage in termsof output on the eight hour compared with the12 hour schedule—for example, there was anincrease in output, of shells manufactured perhour, of 6.5% for an overall average reductionof 20.20 hours per week. A considerable dropin output was found over the last four hours ofa shift, a decrease that was especially promi-nent on 12 hour shifts. Osborne21 concludedthat output was more capable of beingsustained on eight hour shifts. So, both of theseearly studies did not support longer shifts.Fatigue and decreased alertness towards the

end of a 12 hour shift can be a real concern andshould be borne in mind when such systemsare implemented, especially when the job beingworked is highly monotonous and sedentarysuch as monitoring VDU screens, or requires

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continuous heavy physical activity.10 Fatigueand disruption to family and social life havealso been suggested to outweigh benefits in jobsatisfaction and the delivery of care in a hospi-tal setting.22 This is unexpected as one of thesupposed benefits of 12 hour systems is anincrease in time available for social, family, andleisure pursuits. Nevertheless, it is not a uniquefinding, with other researchers reporting simi-lar social disruption on 12 hour systems.23 24

Performance of nurses has been widely stud-ied relative to eight hour and 12 hour rotas.Often, the primary concerns of these studiesare the allocation of human resources and theeVective, economic provision of care. Severalstudies have concluded that there are generallyno diVerences between the two systems infatigue or critical thinking performance,25 26

and one study concluded that 12 hour shiftswould contribute to optimising staYngresources.27 Ugrovics and Wright27 reportedthat the last hour of the first 12 hour shift wasassociated with an increase in fatigue and dropin concentration, although Fields andLoveridge25 concluded that fatigue increasessignificantly over the shift, irrespective of shiftduration. Fatigue and job performance maynot necessarily be linked. Mills et al28 foundthat whereas 12 hour shifts were associatedwith increased fatigue over the shift as well asan increase in performance errors on gram-matical reasoning and medical record review-ing tasks, they did not aVect overall levels of jobperformance.Williamson et al29 investigated the eVects of

12 hour shifts on the work quality and produc-tivity of computer operators. They found that achange from eight hour to 12 hour shifts wasaccompanied by improved wellbeing, especiallypsychological health and reduced tirednessthroughout the work period. There were noapparent costs in terms of job satisfaction orproductivity as a result of the 12 hour rota.Even under conditions of high physical work-load, mining shiftworkers have been reportedto show no diVerences in levels of fatiguebetween eight hour and 12 hour shifts.30

Work eYciency was deemed to have shownan improvement as a result of changing to 12hour schedules in the study of Wilson andRose31 of chemical and petroleum industryshiftworkers. Several contributing factors wereapparent. These were reported to include:improved shift handovers, decreased turnover,and better employee attitudes and communica-tions. A decrease in the number of shiftchangeovers reduced the margins for error withbetter continuity of communication betweenthose starting and those finishing work. An areaof substantial improvement in communicationwas between production and maintenanceworkers. The 12 hour shifts allowed operatorsto make equipment ready for the maintenancecrew and pay more attention to repair or serv-icing because the same crew had to restart theequipment.However, communication may not always

benefit. Breakdown can occur in the interac-tions between shiftworkers and management oradministrators. There can also be communica-

tion problems between shifts after three or fourdays away from work.31 Under eight hoursystems workers can be in the plant for 21 outof 28 days whereas most of the 12 hour rotasstudied required only 14 out of 28 days atwork. On eight hour shifts worker-management overlap at the plant was around10 days per month whereas on 12 hour shiftsthe overlap was five days per month. The longbreaks between work periods were also apotential source of serious diYculties. Thenumber of consecutive rest days ranged fromtwo to eight on some 12 hour schedules. Theformer eight hour schedules provided muchshorter and less frequent time away from theplant, the longest period away being three days,which happened only once every 28 days. Oneconsequence was that on 12 hour rotas therewas a greater reliance on written communica-tions. Shift teams relieving one another afterlong breaks were seldom in contact with oneanother and thus the eYcient transfer of infor-mation about the plant or a process can beimpeded. So, one disadvantage of the increasedpotential for rest and recuperation may be thegreater need for work reorientation on returnfrom long breaks.Evidence from other studies of eight hour

versus 12 hour rotas ties in with this sugges-tion. Compared with those on eight hour shiftsJohnston et al32 reported that nurses on 12 hourshifts tended to be less alert. They also foundthat the 12 hour shift nurses showed increasederrors and faster performance (a classic speed-accuracy trade oV) in a letter cancellation taskand decreased accuracy on a choice reactiontime task. These eVects seem to be acute andassociated with the beginning of the first 12hour shift after a block of rest days; suggestingthe need to consider the appropriate schedul-ing of work tasks in the reorientation periodafter long blocks of rest days on 12 hour rotas.Smiley and Moray33 noted that it is also

important to take into account the sequencingof shifts, the way they rotate, and rest daysavailable as well as overtime practices; all ofwhich may impact on alertness during the shift,performance, and the experience of fatigue.For example, Baker et al34 cited a study whichshowed an interaction between the number ofconsecutive days worked per week over succes-sive weeks and duration of shift. Comparedwith an equivalent number of successive eighthour shifts, there were significant decreases inproductivity on five to seven consecutive 12hour shifts. After four weeks of six or sevenconsecutive 12 hour shifts there were evengreater decrements in productivity. Such find-ings strongly militate against the use of morethan three or four successive 12 hour shifts in acompressed schedule.Todd et al35 and Reid et al36 have also

gathered some unfavourable findings. In oneinvestigation they compared the eVects of eighthour and 12 hour shifts on patient care, workattitudes, and cognitive functioning.35 Theyfound that there was lower quality of care, lessdirect patient care, especially in the later part ofthe day, that nurses’ ability to study decreased,and that 12 hour shifts were not universally

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liked. Acceptance of the longer shifts was gen-erally related to lifestyle considerations. NodiVerences in cognitive performance was foundbetween eight hour and 12 hour shifts. Theauthors suggested that there was no advantagein staYng levels or the reduction of shifthandovers and therefore little justification intheir introduction.Bowers-Hutto and Davis,37 in a study of

nurses on 12 hour shifts, indicated that severalreported impaired judgement, poorer clinicaldecisions, and slower response times when on12 hour shifts. Thompson23 found that nurseson eight hour shifts started their work periodsmore alert than those on 12 hour shifts aftercompleting several simple performance tasks.In a later study Reid et al36 also reported anegative impact of 12 hour shifts on the deliv-ery of patient care. A significant reduction indirect patient care was found in conjunctionwith an increase in unoYcial work breaks. Theresults were suggested to reflect a pacing eVectfor nurses facing 12 hours on duty.Other researchers have suggested that the

eVects of extended shifts may be obscured byother factors—such as motivation. This may bethe reason why, as was noted earlier, fatigueand job performance are not automaticallylinked. Washburn26 reported that althoughfatigue increased towards the end of each shift,performance tended to increase also, possiblyas a result of an increase in compensatory taskeVort. A comparison of police oYcers on eighthour and 12 hour rotas showed large decreasesin subjective alertness from the beginning tothe end of eight hour shifts and 12 hour nightshifts, but no such eVect for 12 hour days.38

The contrast was thought to be a result of theoverwhelming preference of the oYcers for the12 hour system. Another suggestion was thattheir 12 hour day shift ended at 1900 close tothe time of the acrophase in alertness levels andthat the natural increment in alertness com-pensated for any on shift eVects.In a recent paper Rosa18 considered issues

surrounding extended workshifts and theirlinks to excessive fatigue. His arguments werebased mainly on consideration of the relationbetween 12 hour shifts and the build up offatigue. One point of concern is that the poten-tial for an increase in fatigue is particularlysalient when shiftworkers are asked to cover aportion of a subsequent shift, or even doubleup on their shifts to cover for absences or somemutual arrangement with shift colleagues.Twelve hour shifts can even run to 24 hours ofwork in some situations. For example, onoVshore drilling platforms at the transitionfrom a week of 12 hour nights to a week of dayshifts the shiftworker typically worked the last12 hour night immediately followed by a 12hour day before a scheduled break for sleep.39

Thus, it could be argued that extendedworkshifts could have negative consequences,not least in terms of increased fatigue andimpaired job performance.

SummaryThe evidence on the eVects of 12 hour systemson fatigue and job performance is equivocal.

One possible reason for this is that extendedwork periods are popular with shiftworkers asthey compress the working week, and this mayincrease motivation and stimulate greater effortto reduce any possible detrimental eVects ofincreased fatigue. In such cases some benefitsfrom 12 hour systems may appear. However,the characteristics of the shift rota and thedemands of the work performed can have alarge impact on this equality of job perform-ance and the possibility of advantage. There-fore care must be exercised in choosing appro-priate jobs and systems.

SAFETY

Examination of the safety implications of 12hour shifts is particularly keen in relation toindustrial processes or services that hold apotential for considerable public harm such aschemical processing, air traYc control, publictransport, and nuclear power generation. De-spite the increased opportunities for leisuretime on 12 hour shifts, the longer working dayhas the potential to contribute to human errorand accidents at work.18

The fear of greater accident and injury on 12hour shifts is not convincingly confirmed byresearch findings. Northrup40 could find nostudy that conclusively reported such increaseson longer shifts; indeed, he found feweraccidents on 12 hour shifts in the steel industry.Wilson and Rose31 surveyed 50 locations andalso found no reports of increased accidentslinked to the 12 hour shifts. It was understoodthat action had been taken to mitigate thepotential for decreased safety. Managementacted to emphasise preventive safety measuresin response to the fears about possible deterio-ration in safety performance. Employees alsorecognised the possible eVects of fatigue andthese concerns led to greater conscientiousness(it is, however, appropriate to question the lon-gevity of the eYcacy of these preventativeactivities). Several sites reported having theirlongest periods without injury based lost time:a range from two to 10 million man hours hadbeen worked without a lost time injury after theintroduction of 12 hour shifts. One plantreported a 60% decrease in recorded injuriesbut the downside to this was a 55% increase inhazardous incidents which did not lead toinjury (although this was also linked tomanagement’s increased attention to reportingof hazardous conditions).In one of few studies that have made an

attempt at a direct comparison Lees andLaundry41 reported a study of reported acci-dents in a company that had changed fromeight to 12 hour shifts. Their overall conclusionwas that minor and moderate injuries (withoutlost time) decreased significantly in the 10years after the change to the 12 hour shiftregime compared with 10 years of accidentdata before the change. More recently, Pollocket al42 reported a comparison of injury patternson eight and 12 hour shifts in a petrochemicalplant and a fertiliser producing company. Theirbroadly based results showed that neither com-pany showed a significant change in frequen-cies of injuries after implementing the 12 hour



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shift. Where severity was examined it wasreported that after the change to 12 hour shiftsthere was a higher percentage of more seriousinjuries and a lower percentage of minorinjuries on 12 hour shifts in the petrochemicalcompany. No diVerences emerged for the ferti-liser plant. The authors suggested that 12 hourshifts may have diVerential eVects in diVerentcompanies because of the nature of the work,the characteristics of the workers, and the envi-ronment. Part of the explanation may also lie ina change to a more rigorous safety culturewithin an organisation.Studies in nuclear power plants which have

converted from eight hour to 12 hour shiftshave found no significant decreases in produc-tivity nor increases in operator error after thechange.43 Smiley and Moray33 reported thatafter a seven year period of recording allincidents at a nuclear plant that had adopted12 hour shifts there was no evidence of adeleterious impact. A possible factor involvedin improvements after a switch to 12 hour shiftsis the reduction in the number of shift hand-overs that occur. Indeed, the company manage-ment thought that handovers between twocrews were an improvement on those for threecrews on an eight hour system. Another studyat an experimental nuclear reactor examinedthe eVects of changes from an eight hour to a12 hour rota.44 Although no night shift datawere collected it was concluded that the 12hour day shift was a reasonable alternative toeight hour day shifts with respect to safety andjob performance. There was a 25% decrease inerror in completing operational logs after themove to 12 hour day shifts. Operators wereslightly less alert at the end of their 12 hourshifts compared with the eight hour shifts butthere was a significant diVerence between thetwo schedules of the performance integrity ofthe plant on 12 hour shifts.Moreover, supervi-sors and most operators reported greater workeVectiveness associated with the 12 hour shifts.The 12 hour shifts were considered betterbecause only 12 hours was spent away from theplant between successive shifts. Thereforeoperators remained job oriented to a greaterextent than they would with a 16 hour breakbetween successive shifts. Communication atshift handover was reported to be betterbecause on the 12 hour rota shift teams leavingwork handed over to the same team it receivedthe plant from 12 hours earlier. In such a highpotential hazard industry, optimising the flowof communications between shift crews isclearly crucial. Nevertheless, diYculties canarise as shiftworkers can become out of touchwith conditions at the plant. This was high-lighted by greater impairments in shift hand-overs after a block of seven rest days on the 12hour system compared with after a block offour rest days on the eight hour system.Not all investigators have come to positive

conclusions about 12 hour systems. For exam-ple, Kelly and Schneider45 examined theperformance records of operator error and per-sonal accident at a nuclear power plant relativeto eight hour and 12 hour shifts. Their riskassessment suggested a 70% increase in the

probability of an incident with public implica-tions to be associated with the operation of 12hour shifts. Despite these dire predictions andcontrary to the report in the article45 the plantimplemented a 2-3-2 pattern of 12 hour shiftsand it has operated these shifts ever since (per-sonal communication Baker TL). Safety wasnot compromised and incidents relevant to thepublic did not increase. Similarly, Rosa andColligan46 considered a decline in alertness andperformance during the last few hours of a 12hours shift and disruption to subsequent sleepepisodes problematic at a nuclear plant. How-ever, this is not uncommon, even towards theend of eight hour shifts, and in this case it wasconcluded by the management that the opera-tional integrity of the plant remained un-changed under the 12 hour schedule. Theincreased number of rest days on 12 hour sys-tems may compensate in the long run for acutesleep disruption.Rosa47 also examined control room operators

at a continuous processing plant. After sevenmonths on a 12 hour schedule, decrements inalertness and performance as well as a mild tomoderate sleep debt were found comparedwith the original eight hour system. A three tofive year follow up of these shiftworkers showedpersistent decrements in performance andalertness attributable to 12 hour shifts and areduction in sleep duration after night shifts.There was, however, little deterioration in per-formance or alertness across the working week,which suggested day to day recovery from theextended work period. Workers may be moti-vated to tolerate a 12 hour system because ofthe potential social gains the schedule allows.47

In another study at a natural gas facility Rosaand Bonnet48 reported that after 10 monthsadaptation to a new 12 hour schedule therewere decrements in performance and alertnessattributable to the last hours of the extendedwork shift. There was also disruption to sleepacross the working week that was mostpronounced for the 12 hour night shifts. As aresult it was suggested that extra caution beexercised in the scheduling of critical activitieson 12 hour shifts, especially at night.The main concerns for safety and human

performance and guidelines from the USNuclear Regulatory Commission were re-viewed by Gould49 who discussed the safetyimplications of 12 hour shifts in nuclear powerplants. Several recommendations were sug-gested. Firstly, 12 hour schedules should bearranged so that overtime is covered withoutholding over a 12 hour shiftworker or calling ina worker who had fewer than three rest daysafter four consecutive 12 hour shifts. Secondly,the round trip to and from work should be lessthan three hours a day because any longer thanthis would reduce the opportunity for adequatesleep. Thirdly, active eVorts should be made toreorient the workers after breaks of longer thanfour days to help counter the forgetting factor.Fourthly, to help reduce fatigue shorter andmore frequent breaks may be more appropriatethan long ones. It was also reported that 12hour shifts have been implemented at nuclearpower plants with no evidence of a reduction in



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job performance nor increase in error rates.Moreover, he stated that there could beimprovements to communications at shifthandover as well as increases in job satisfaction.

SummaryThere seems to be no conclusive evidence thatextended work shifts compromise safety fromthe point of view of increased accident rates, orfrom reduction in job performance, or increasein error rates. However, in part this may be dueto increased emphasis on preventive safetymeasures in response to the fears about possi-ble deterioration in safety from both manage-ment and employees. It is therefore appropriateto question the longevity of the eYcacy of thesepreventive activities. Nevertheless, providingthat adequate measures are taken it seems that12 hour shifts do not automatically induce asignificant decrement in safety.

SLEEP, PHYSICAL HEALTH, AND PSYCHOSOCIAL

WELLBEING

Sleep and general wellbeing have also beenexamined for 12 hour systems. Wilson andRose31 noted that shiftworkers reported feelingat least as good, or better, emotionally andphysically on 12 hour shifts than on eight hourrotas. Publications on 12 hour schedules oVersome support for this contention.Lees and Laundry50 examined archival evi-

dence on workers’ health over a 10 year periodon eight hour shifts followed by a 10 yearperiod on a 12 hour rota, within a steel plant.The workers reported fewer headaches andproblems with digestion and alcohol misuse onthe 12 hour schedule. However, there was nodiVerence between the eight and 12 hour rotasof feelings of general malaise or nervous condi-tion. Several other studies in a range ofoccupations have reported little or no deleteri-ous eVect of 12 hour shifts on factors such asphysical and psychological wellbeing, stress,and tiredness.30 50 51 Tucker et al52 found that,except for levels of alertness, there wererelatively few diVerences between eight and 12hour systems in health and wellbeing, disrup-tion to sleep, and social life. What diVerencesthere were tended to favour (although notexclusively) the 12 hour system. They con-cluded that compression of the working weekinto fewer, longer shifts need not necessarily beproblematic for the workers and there may beadvantages to working 12 hour shifts.JaVe et al53 compared three groups of

petrochemical workers (dayworkers, shiftwork-ers on an eight hour advancing (NAM) system,and workers on a 12 hour rota). The eight hourshiftworkers had greater problems with sleepquality, and health complaints, and expressedleast satisfaction with time available for per-sonal and family pursuits. The sleep qualityproblems were attributed to the quick returnsin the rota and the direction of rotation of theschedule, which mitigates against the biologicalclock’s natural tendency to delay and thusfavours delaying systems. JaVe et al53 concludedthat other features of the shift schedule aremore critical than duration oV duty in explain-

ing disruption to sleep, health, and social life(see also Knauth54).In their study of police oYcers Peacock et al38

found that the 12 hour system providedopportunity for better sleep habits than aneight hour system because for 75% of the 12hour eight day shift cycle implemented, a nor-mal day-night sleep pattern could be main-tained. For physical fitness, the same studyreported that oYcers on the 12 hour shifts wereable to achieve higher workloads with lessphysiological strain. OYcers were found to befitter on the 12 hour rota, possibly because ofbetter sleep patterns, greater regularity ineating habits, and more opportunity for recrea-tional activities. Gadbois and Prunier55 studiedcustoms oYcers and found that those working12 hour shifts had better health and sleep thandid colleagues working six hour shifts. Oneconfounding factor, however, was diVerencesin the kinds of tasks performed. Iskra-Golec etal56 reported a negative impact of 12 hour shiftson wellbeing but suggested that the eVects maybe moderated by the nature of the job beingdone.Chan et al57 studied the health complaints,

blood pressure, sleep, and sickness absence offemale electronics workers who had worked 12hour shifts for over a year. No serious healthproblems were evident in these shiftworkers.The only significant diVerences in symptomprevalence between the 12 hour shiftworkersand a control group of eight hour day workerswere complaints of tiredness and headaches.Hadjiolova et al58 examined cardiovascular

strain and shiftwork tolerance in operators inthe control room of a chemical plant on 12hour shifts. After job analysis and time budgetassessment the authors concluded that theoperators’ jobs were suitable for the extendedworkday schedule. Indeed, the operatorsstrongly preferred 12 hour shifts and even afterthree years of the rota their subjective ratings ofphysical and mental demands did not diVerfrom those before the schedule’s implementa-tion.A recent study of the attitudes of nurses

towards eight hour and 12 hour shifts hasfound some advantage of eight hour shifts.59

This research considered the social and familyconsequences for nurses working on these twoforms of shift system. Over 40% of each groupreported low satisfaction with their scheduleand a desire for its change. Eight hour shiftswere viewed more favourably because of lowerlevels of work strain, better continuity ofservice, and less disruption to health, family,and leisure life. The impact of schedule type onfamily and social life was a main moderator ofsystem satisfaction and attitudes towards achange in rota. It was concluded that neithertype of shift system was very attractive to theparticipants because both schedules interferedwith family and social responsibilities. How-ever, there seemed to be negative eVects of 12hour shifts on social and family life, preciselythose areas that are argued to be improved bysuch a system.22 23 The authors noted that theseeVects were indirect. It was suggested that theeVects were due to a need for greater recovery



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(from higher reported adverse health eVectsand poorer subjective health state on 12 hourshifts), which resulted in the erosion of freetime during rest breaks.Another area of possible diYculty occurs

when considering the threshold limits forexposure to environmental stressors and vari-ous chemicals in the workplace. Legislation, forexample, is normally based on eight hourexposure limits. Recently, Knauth54 highlightedthis point in recommendations for implement-ing a shift system and expressed concern aboutthe potential harm of extended exposure totoxic chemicals, and other adverse workconditions—such as noise or heat—that work-ing 12 hour shifts could entail. In this respectresearchers have suggested that work condi-tions and the type of work being done willinteract with the schedule and exposure limitsshould be a main consideration when thinkingabout implementing 12 hour shiftsystems.54 60 61

Reynolds et al62 assessed a hearing conserva-tion programme which had been instigatedunder a three × eight hour system but nowoperated under 12 hour shift conditions. Froman occupational health position it was foundthat the existing hearing conservation pro-gramme was at best marginal in eVect irrespec-tive of type of schedule. It was concluded thatthe 12 hour system had no impact on the levelof eVectiveness of the programme.Individual diVerences in various personal,

social, and environmental factors will almostcertainly moderate the impact of a shiftschedule.39 63 This point is illustrated in thereport of one study of chemical industry shift-workers who found that performance on reac-tion time, reasoning, visual search, and tappingtasks was poorer on 12 hour than eight hourshifts.64 The authors indicated that the resultswere confounded by the diVerences in capabili-ties of the workers on each rota.Rosa18 suggested several rarely studied fac-

tors that might exacerbate the potential fatigu-ing eVects of longer work periods. These com-pounding influences include workplacestressors—such as workload, noise, exposure totoxic chemicals, heat—organisational issues—such as staYng levels, nature of job tasks, andrest breaks—as well as commuting time andpressures due to domestic and social responsi-bilities outside work. His overall message is oneof cautious introduction of 12 hour systems inconcert with careful evaluation of their impact.

SummaryMuch of the evidence suggests that shiftwork-ers do not have great problems with sleep,health, and wellbeing when working 12 hourcompared with eight hour shifts and may evenshow improvements in these areas. The situa-tion is not entirely straightforward and consid-eration has to be given to working conditionsand the nature of the job, particularly work-place stressors.

SYSTEM IMPLEMENTATION, ATTITUDES,PREFERENCES, AND MORALE

The way a new shift system is introduced to aworkforce has a substantial impact on itsdegree of acceptance. Johnston et al32 for exam-ple, reported that nurses in their study dislikedthe 12 hour system and this attitude may haveinfluenced subjective reporting. However, theoverall conclusion was that there were nodeleterious eVects of the 12 hour rota in termsof tiredness and stress compared with eighthour shifts. In contrast, the success of a 12 hoursystem for nurses who had worked it for threeyears was considered to be dependent on howand why the schedule was implemented.23 Forexample, satisfaction with a 12 hour rota wasreported where it was introduced to helpreduce commuting. In a study on the changefrom an eight to a 12 hour rota at an oil refin-ery Conrad-Betschart65 pointed out that theshift system design and implementation proc-ess had been a neglected area of research. Thestudy showed that employee participation innegotiating the system and changes to condi-tions related to shiftwork—such as provision ofan extra break during 12 hour shifts andimproved meal facilities at night—was criticalfor the acceptance and positive eVects of thenew schedule. The author argued that thefindings confirmed those of other research intothe influence of direct participation in theorganisation of work scheduling.66 67 There willinevitably be diYculties in getting the fullconsent of a shift workforce, because nosystem can suit all preferences. Nevertheless,Conrad-Betschart65 suggested that these prob-lems will be transitory if all relevant stakehold-ers have the opportunity to be involved in theprocess.Considering the apparent popularity of 12

hour systems, Smith et al68 posed the questionof whether 12 hour shift systems can be a solu-tion to some of the diYculties experienced byshiftworkers in a study of police oYcers. FewdiVerences were found between 12 hour andeight hour shift groups after a six month trialperiod. The diVerences that emerged seemedto be linked to the nature of the implementa-tion of the 12 hour rotas. At one site a flexible12 hour system was implemented which gavethe opportunity to negotiate the timing of workto fit personal circumstances. There was also alater start to the day shift (0715) on the flexiblesystem which seemed to contribute to im-proved sleep quality after the day shift and themaintenance of alertness on shift. In contrast,these variables decreased for those on the rigid12 hour system with its earlier start (0600).Daytime sleep durations after night shifts,however, did not show the same benefits. Apossible explanation is that coming oV thenight shift later (at 0715) on the flexible rotameans that by the time the shiftworker getshome and goes to bed, the body clock is moregeared up for wakefulness and activity than itwould otherwise be after the 0600 end of shift.Psychological health was maintained at a stablelevel on the flexible system but deteriorated inoYcers on the rigid system. The situation wasnot simply one of eight versus 12 hour shifts



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but rather of how features of a rota are changedand how a new shift system is implemented.Flexibility and opportunity to exert some con-trol over working time seems to be an increas-ingly important factor in how a rota is acceptedby a workforce and impacts on individualpeople.69

Suter70 commented on the introduction of 12hour shifts at an American nuclear powerplant. Cooperation with those directly aVected(including union representation) was consid-ered vital to the success of implementation.Indeed, shiftwork research has suggested thatemployee participation and sense of control iscrucial.69 Wilson and Rose31 reported favour-able responses to 12 hour rotas as a result ofemployees choosing the systems—generallyperceiving the benefits to outweigh the costs.At one plant where the schedule was imposed,worker reactions were negative to such anextent that the old three × eight hour schedulehad to be reinstated. Peacock et al38 concludedthat there were no disadvantages of a 12 hoursystem with an eight day cycle when comparedwith an eight hour system with a 12 day cyclefor sleep patterns, physiological and psycho-logical eVects, or subjective measures ofalertness. They reported overwhelming sup-port for the 12 hour system from variousoperational and social standpoints but did sug-gest the need to monitor whether attitudesmight change in the longer term once the hon-eymoon period was over. Tepas,71 on the otherhand, has suggested that condensed workweeks may gain in popularity as experience ofthem in the work sector increases.In many of the studies on 12 hour schedules

the shiftworkers tended to prefer them to eighthour shifts. The main rationale for preferring12 hour rotas is that compressed workingweeks allow more free time away from workand this can have positive spill-over eVects ondomestic life. For example, Barton-Cunningham72 found that younger married 12hour shiftworkers were more satisfied withtheir family relations. Others have commentedon this aspect of compressing the workingweek27 (see also earlier sections on fatigue andsleep, physical health, and psychosocial wellbe-ing). One caveat to this position (untested atthis point) is that some shiftworkers may find itdiYcult to adjust to spending more time withtheir families. Research has also highlighted adiVerence in perceived benefits between shift-workers and their partners. Wallace et al24

reported that the partners of shiftworkers whohad changed from an eight hour to a 12 hoursystem saw no particular advantage in thechange for family and personal life, whereas theshiftworkers themselves were strongly in favourof the change to 12 hour shifts. Attitudestowards 12 hour shifts may also vary accordingto the nature of the work task being carried out,level of responsibility, and working environ-ment. Smiley and Moray33 noted that mainte-nance personnel showed a greater preferencefor 12 hour shifts than control room supervi-sors. One reason for this was the supervisors’feeling of an increased burden of responsibilityfor plant safety on longer shifts. Another argu-

ment for the diVerence in attitude was thatmaintenance crews’ work was more varied, selfpaced, and involved more physical activitywhereas operators and supervisors performedlargely sedentary monitoring tasks.

SummaryIt has been suggested that part of the reasonthat 12 hour systems have not been shown tohave a detrimental eVect on job performance orsafety is that in general the shiftworkers are infavour of the systems and may make eVorts toaccommodate them. The corollary to thishypothesis must be that the way the system isimplemented is of great importance to the atti-tudes, preferences, and morale of the workforce and ultimately to the success of theimplementation. It is important to involve thework force in the design and implementationprocess so that not only will it have theirsupport, but the diVerences in their work andattitudes to that work will be registered.

ABSENTEEISM AND TURNOVER

Given the above arguments it would be reason-able to assume that those shiftworkers whohave a positive attitude towards their workschedule would be less likely to be absent fromwork, leave work, or change to a day job withinwork. Contrary to prevailing expectationIvancevich73 found no diVerence in rates ofabsenteeism between compressed workingweeks (four to 40) and standard five day weeks.Considering sickness absence and frequency ofmedical consultations, Nachreiner et al74 alsofound no significant diVerences between eightand 12 hour shiftworkers. Others have notedthat decreases in absenteeism were thought tobe one of the benefits of introducing 12 hourschedules but improvements were only foundin six out of 50 sites examined and only one ofthese improved significantly.31 There seemed tobe a “floor eVect”, because the companiesreported no real problems with sicknessabsence before the change to 12 hour rotas.The consensus was that 12 hour shifts did notincrease rates of sickness absence. A similarconclusion was drawn more recently byWilliamson et al.29 They reported that a changeto 12 hour shifts for computer operators had nosignificant ill eVects on factors related topersonnel—such as absenteeism and staVturnover. Turnover of both job leavers andtransfer from shiftwork to day work has beenreported to remain stable on 12 hour systems.Wilson and Rose31 reported general improve-ments on both counts after the introduction of12 hour rotas at 50 industrial plants. Employeemorale showed improvements in every locationsurveyed and it was viewed as the main benefitaccrued by moving to 12 hour shifts. Shift-workers thought that many of the onerousaspects of their work had been removed andthey were more satisfied with their employerand schedule.Enthusiasm for the rotas may, however,

divide along age lines. Older workers had lessfavourable attitudes and lower support for the12 hour rotas. Age may have also been aconfounding factor in an article comparing



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health issues and absenteeism in eight and 12hour shiftworkers.75 The findings suggestedthat 12 hour workers (who were older) hadhigher rates of absence, higher injury rates, andhad higher levels of sickness and intoxication.Older people tend to have more ill health and itis, perhaps, not surprising that an older groupof shiftworkers experienced greater diYcultieswith their 12 hour shift rota. However, in amore recent study Keran et al76 reported nointeraction between age and shift duration onphysical and psychological measures. De-creased turnover was also a feature of the posi-tive outcome of a change to 12 hour shifts inthe study of Conrad-Betschart.65 The studyreported that an increase in the supply ofapplicants was a persuasive factor in thepermanent introduction of the 12 hour system.Interestingly, a less positive evaluation of the 12hour rota was found among older workers.65 Itwas suggested that this was, in part, the resultof the new schedule making some forms ofmoonlighting more diYcult. Finally, it may bethe case that any decrease in absenteeismfound on a 12 hour regime is an artefact ofhaving fewer workdays to be absent from, apoint well made by Hoekstra et al.77 This, ofcourse, could also account for reported dropsin reporting of accidents41 and thus apparentlyimproved safety.

SummaryIn general absenteeism and turnover did notincrease with 12 hour systems. However, olderworkers may find it more diYcult to deal withthose rotas.

OVERTIME AND MOONLIGHTING

Work eVectiveness, safety, and wellbeing are atrisk of deterioration as a result of overtimepractices compounding potential diYcultiesarising from a shift schedule. A primaryconcern is the possible impairment to alertnessand performance that may be associated withfatigue from excessive hours on duty. AlthoughRosa18 points out that overtime has rarely beenassociated with safety issues or accidents, theextension of the working day through sched-uled and unscheduled overtime is an area thatrequires investigation from both a health andsafety perspective.There may be problems with adequate staV-

ing despite a potential reduction in unsched-uled overtime costs as a result of moving to 12hour schedules. For example, there may be dif-ficulty in filling unscheduled shift vacancies.Contributing factors could include understaV-ing, existing overtime schedules, diYculty incontacting employees during their rest days,and the scheduling of safety meetings for thefirst day of a block of rest days. One plant in thestudy of Wilson and Rose31 reported a decreasein unscheduled overtime from around 7% onthe old eight hour schedule to just over 4% inthe first year of the 12 hour shifts. This reduc-tion was attributed to a combination of fourcauses. Firstly, absenteeism decreased; sec-ondly, 12 hour shifts generally eliminated theneed to hold an operator over from their previ-ous eight hour shift when plant downtime

lasted nine to 10 hours; thirdly, on eight hourshifts a long shutdown meant three shifts wereinvolved whereas there were only two on the 12hour rota—reducing the possibility of errorsand subsequent overtime; and lastly, additionaloperators were employed to facilitate trainingwhich meant less need to call employees in towork overtime. Nevertheless there is potentialfor diYculties to arise should shiftworkers haveto be called in from their rest days or berequired at the end of a 12 hour shift to carryover into the next to cover an unexpectedabsence.A report for the Nuclear Regulatory Com-

mission (NRC) in the United States consid-ered the issue of overtime.34 It was suggestedthat when overtime is scheduled its combina-tion with longer shifts may result in shiftwork-ers’ sleep and recovery time becoming de-pleted. More of a problem is unscheduledovertime which is potentially more disruptiveas it increases the diYculty in organising sleep,recovery, and social regimens.18 It has beensuggested that although 12 hour rotas mayresult in less overtime per employee per yearproblems could arise because overtime proce-dures have often been designed assuming aneight hour shift system.34 Where there is norelief crew overtime is typically covered by call-ing in operators on their days oV, to complywith NRC recommendations on daily limits onhours worked. A consequence of this is thateVort must be put into protecting rest days andevenly distributing overtime. One recom-mendation is for overtime to be voluntaryrather than mandatory, but there are limita-tions to this option. Shiftworkers may underes-timate or ignore the number of hours they havebeen awake before working the overtime. Someshiftworkers will inevitably volunteer for, andwork excessive hours. Others may completelyrefuse to work any overtime.The disseminationof more information about the eVects of shift-working and long work hours would enableinformed choices to be made about workingovertime. This, combined with organisationalprocedures to check imbalances in the distribu-tion of overtime, could contribute to a loweraverage impairment for shiftworkers as awhole. The implications of overtime for safetyat nuclear power stations were presented morerecently by Baker et al78 who examined therelations between overtime worked by opera-tors, technical staV, and maintenance staV andsafety incidents. The analysis suggested thatsafety incidents were most strongly associatedwith the average annual overtime worked bycontrol room operators. Operator fatigue as aresult of this longer working time was sug-gested to have contributed to safety incidentsrather than a reduction in operational integrityof the plant because the overtime activity of thetechnical and maintenance crews did not showany strong association with the incidents.A further implication of larger blocks of days

oV from work is the opportunity for shiftwork-ers to moonlight. Financial motives may welltempt shiftworkers to take up a second (andeven third) job during the period designated forrest and recuperation. Twelve hour shifts could



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contribute to moonlighting through the avail-ability of greater amounts of leisure time andthe potential for boredom as a by product ofthe longer periods away from work. Colliganand Tepas79 found that 25% of 12 hour systemshiftworkers were moonlighting. Their concernwas that moonlighting may involve the exten-sion of work activity beyond 12 hours andinevitably results in many people returning towork, possibly the night shift, already tiredfrom their extramural activities.

SummaryThe issue of overtime is an area of someconcern with 12 hour shift systems. This isbecause of the potential impairment to alert-ness and performance that may be associatedwith fatigue resulting from excessive hours onduty. Therefore, care must be taken to designovertime staYng around the system involved,so that, for example, procedures left over fromeight hour systems are not used. Another prob-lem is that the opportunity for moonlightingmay also increase with a 12 hour system againwith possible consequences of fatigue resultingfrom excessive hours of work.

DiscussionExisting knowledge about the extent, nature,and eVects of shiftwork allows several generalpoints to be made. Firstly, for better or worse,shiftwork is here to stay. For example, healthcare, supply of food, and power generation willalways be required 24 hours a day. Secondly, toprovide continuous cover some form of shiftsystem must be employed. This often results indisruption to the worker because it conflictswith our evolutionary development as diurnalcreatures (primarily active during the day,inactive at night). Thirdly, a shift system canhave a considerable impact on the people hav-ing to work it. This impact may manifest itselfin terms of sleep, health, and social disruption,as well as on job performance and safety.Fourthly, these potential eVects of shiftworkwill depend to varying degrees on the nature ofthe job—for example, sedentary, monitoring ofa control panel, or active physical work—personal characteristics of the individual—forexample, a favourable predisposition to nightwork or eVective coping skills—and features ofthe system worked—for example, direction andspeed of rotation, duration of shifts, andnumber of consecutive shifts of the same kind.The primary focus of this paper has been theimplications of 12 hour shifts for both the indi-vidual worker and the organisation.This paper has considered the influence of

duration of the shift on shiftworkers’ generalwellbeing and eVectiveness. Evidence on therelative impact of the two main organisations ofshift duration within a shift system has beenpresented—namely, the eVects of rotating eighthour and 12 hour shift rotas on sleep, health,and performance, as well as implications for theorganisation. Perhaps the most important pointto be made is that the research findings arelargely equivocal with both costs and benefitsoccurring as a consequence of working 12 hour

shifts.10 The arguments for and against imple-mentation of such shifts can be roughly catego-rised into two broad areas.On the positive side, there is a motivational

and attitudinal aspect to the acceptance of 12hour rotas, and self rated stress levels may beconsiderably reduced. Also there can be highemployee acceptance and satisfaction with thesystem, reduced staV turnover, and improvedoV duty durations of sleep and sleep quality.Perceived advantages of 12 hour schedules caninclude improvements in several areas of shift-workers’ lives. Family relations may be en-hanced because there is more time for familyand friends and more frequent weekends oV.Thus, social activity and involvement can beincreased. Fewer journeys to and from workcan mean savings in the costs and strain oftravel. Both physical and psychological wellbe-ing may benefit. Shiftworkers have reportedbetter appetite and a sense of feelinghealthier.29 38 53 The 12 hour system can alsohave benefits in better communication at shifthandovers. It is also likely that the acceptanceof a 12 hour rota by a workforce already hold-ing a favourable attitude towards it will beincreased by the opportunity for direct partici-pation in the organisational decision andimplementation process.On the negative side, the main concerns are

about eVects of fatigue and safety. Although 12hour rotas tend to be popular with shiftwork-ers, where there is a high workload their intro-duction should be the subject of carefulscrutiny because of the potential for a build upof fatigue.18 64 The follow up study of Rosa47

which showed persistent alertness and per-formance decrements on 12 hour shifts, addsto the note of caution. Indeed, it should beemphasised that a 12 hour shift does notusually equate with only being active for 12hours. There can be considerable extension ofthe person’s time awake on either side of theshift as a result of factors such as sleep curtail-ment, a long journey, meetings arranged after ashift, and scheduled and unscheduled over-time. There are other disadvantages. Shift-workers may have to spend long hours on theirfeet. There may be limitations to educationaland other leisure activities. Days oV canbecome boring if a partner works and spendingmay increase during longer rest breaks. It isalso possible that spouses may not like 12 hourschedules even when shiftworkers stronglyfavour them.In their study of 50 plants where the range of

time that 12 hour rotas had been implementedwas four months to five years, Wilson andRose31 noted that the eVects of longer termexposure to extended working days had notbeen thoroughly investigated in any systematicway. Many studies have only examined thereplacement of eight hour schedules by 12 hourones in the short term (although there areexceptions—for example, Lees and Laundry41).There is a need for research that examinesthe impact of 12 hour shifts once the euphoriaof the honeymoon period has ended.18 It ispossible that as workers become used to the 12



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hour rotas, new problems will emerge in thelong term.One point should be made about some of the

papers used for this review—for example, thearticle by Wilson and Rose31 which has beencited several times during the course of thereview. It must be said that the paper is largelyanecdotal and unsystematic in its approach.Noapology is made for using it as it still containsmuch useful information. It must not beforgotten that a main purpose of research is tohelp form decision making and policy that willhave a very real impact on some peoples’ lives.If we do not listen to their accounts we not onlymiss out on a large body of information whichcould inform our theoretical positions but wealso lose the opportunity to externally validatethe conceptual models on which we base thosetheoretical positions. Nevertheless, althoughsuch papers are valuable it should be recog-nised that they can only progress the argumentso far.Most evidence cited stems from field re-

search which attempts to instil a more system-atic and rigorous approach but still maintainsecological validity. Although providing insightinto the possible costs and benefits of operatingthe shift systems, acknowledgement must alsobe made of the shortcomings of that mode ofresearch. Very often, because it is a real worldsituation, the researcher has little if anyinfluence on many possibly confounding fac-tors. Achieving and maintaining parity betweencomparison groups on all but the variables ofinterest is almost invariably compromised. It is,and has to be, research of a most pragmaticnature. This raises the question of laboratorystudies examining eight hour versus 12 hourshift eVects.Extraneous influences such as temperature,

workplace lighting, and work scheduling werecontrolled in a simulated control room studycomparing alertness, mood, performance, andoV duty sleep on eight and 12 hour night shiftsreported by Baker.14 34 Commuting was elimi-nated, the exposure to daylight was limitedduring oV duty hours, there were optimal oVshift sleeping conditions, and there were nofamily or social responsibilities to attend to.The main findings of this study were as follows.The 12 hour group tended to show longerresponse times but greater accuracy than theeight hour group on several short durationcognitive performance tests. Subjects workingeight hour shifts showed greater accuracy onone performance test. No diVerences emergedbetween the groups when operating a simu-lated process control, or in speed of respondingto alarm signals. There were also no significantdiVerences in terms of sleep quality or durationor in sleepiness measured by periodic multiplesleep latency tests, nor were there diVerences inself rated alertness, tiredness, or eVort. Undercontrolled conditions neither cognitive per-formance, alertness, nor simulated work taskswere notably impaired on the 12 hour nightshifts. Both eight hour and 12 hour night shiftprotocols showed a night of shift eVect whichsuggested that subjects adapted to night dutyafter three to four shifts. This was reflected by

improvement in subjective mood and alertnessratings, and speed of performance.The general conclusions from this review

must be that, on the basis of the evidence,especially that stemming from systematicapplied research, it is not tenable to presume12 hour shifts to have a systematic detrimentalimpact. Indeed, this view was strengthened bythe evidence in papers presented at a recentinternational symposium on night and shift-work held in Finland. Many papers replicatedreports of there being advantages to 12 hoursystems compared with eight hour rotas interms of improvements to family and social life,no increases in accidents, injuries, or near missincidents on 12 hour shifts, greater satisfactionwith rotas, no major impact of longer shifts onsleep or sleepiness, greater ease of fallingasleep, and feeling rested after sleep, little or nohealth outcome or diVerences in psychologicalwellbeing between the schedules and even areduction of working hours on 12 hourshifts.80–83 The picture was not wholly positive,with some papers suggesting the need for cau-tion in jobs with heavy physical demands,84 andothers raising concerns about the exacerbatingeVects of workload, aging, and overtime onchronic exposure to longer work shifts.85 It istherefore evident that there are many questionsstill to be answered about the possible longerterm eVects of working 12 hour schedules. Theobvious requirement in the present position isfurther well controlled, longitudinal researchinto the long term impact of 12 hour shift sys-tems.

A summary table can be obtained from the correspondingauthor on request.

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tively in the order in which they are first men-tioned in the text by Arabic numerals on theline in square brackets on each occasionthe reference is cited (Manson[1] confirmedother reports[2][3][4][5]). In future ref-erences to papers submitted to Occup EnvironMed should include: the names of all

authors if there are three or less or, if there aremore, the first three followed by et al; the titleof journal articles or book chapters; the titles ofjournals abbreviated according to the style ofIndex Medicus; and the first and final pagenumbers of the article or chapter. Titles not inIndex Medicus should be given in full.Examples of common forms of references

are:

1 International Committee of Medical Journal Editors.Uniform requirements for manuscripts submitted tobiomed journals. JAMA 1993;269:2282-6.

2 Soter NA, Wasserman SI, Austen KF. Cold urticaria:release into the circulation of histmaine and eosinophilchemotactic factor of anaphylaxis during cold challenge.N Engl J Med 1976;294:687-90.

3 Weinstein L, Swartz MN. Pathogenic properties of invad-ing micro-organisms. In: Sodeman WA Jr, Sodeman WA,eds. Pathologic physiology, mechanisms of disease. Philadel-phia: W B Saunders, 1974:457-72.



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doi: 10.1136/oem.55.4.217 1998 55: 217-229Occup Environ Med

L Smith, S Folkard, P Tucker, et al. hour and 12 hour shift systems.Work shift duration: a review comparing eight

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