Chapter43-HoursofWorkHOURSOFWORKPeterKnauthShiftworkisworkscheduled,eitherpermanentlyorfrequently,outsidenormaldaytimeworkinghours.Shiftworkcanbee.g.,permanentworkatnight,permanentworkduringtheevening,orworkhourscanhavechangingassignmentpatterns.Eachtypeofshiftsystemhasitsadvantagesanddisadvantages,andeachisassociatedwithdifferingeffectsonwell-being,health,sociallifeandworkperformance.Inthetraditionalslowlyrotatingshiftsystems,shiftschangeweekly;thatis,aweekofnightshiftsisfollowedbyaweekofeveningshiftsandthenaweekofmorningshifts.Inaquicklyrotatingshiftsystemonlyone,twooramaximumofthreeconsecutivedaysarespentoneachshift.Insomecountries,liketheUnitedStates,shiftslongerthan8hours,inparticular12hours,aregaininginpopularity(Rosaetal.1990).Humanbeingshaveevolvedasessentiallydiurnal;thatis,thebodyismainly“programmed”towardsdaytimeworkperformanceandfornight-timerecreationandrest.Internalmechanisms(sometimescalledthebodyorbiologicalclock)controlthephysiologyandbiochemistryofthebodytofitinwitha24-hourenvironment.Thesecyclesarecalledcircadianrhythms.Thedisruptionofcircadianvariationsinphysiologicalfunctioncausedbyhavingtobeawakeandatworkatbiologicallyunusualhours,aswellastosleepduringthedaytime,isoneofthemajorstressesassociatedwithshiftwork.Despitethewidespreadassumptionthatdisturbancesofthecircadiansystemmayresult,overthelongrun,inharmfuleffects,theactualcause-effectrelationhasbeendifficulttoestablish.Despitethislackofabsoluteproof,itiswidelyacceptedthatitisprudenttoadoptshiftsystemsattheworkplacethatminimizelong-lastingdisruptionofcircadianrhythms.CombinedEffectsofWorkplaceFactorsSomeshiftworkersarealsoexposedtootherworkplacehazards,suchastoxicagents,ortojobswithhighmentalloadsorphysicaldemands.Onlyafewstudies,however,haveaddressedtheproblemscausedbythecombinationofshiftworkandunfavourableworking,organizationalandenvironmentalconditionswherethenegativeeffectsofshiftworkcouldbecausednotonlybythephasedifferencebetweencircadianrhythmsandlivingconditions,butalsobytheadversenegativeworkingconditionsthatmaybecombinedwithshiftwork.Avarietyofworkplacehazards,suchasnoise,unfavourableclimaticconditions,unfavourablelightingconditions,vibrationandcombinationsofthese,cansometimesoccurmoreofteninthree-shiftsystems,irregularsystemsandnight-shiftsystemsthanintwo-shiftsystemsordaywork.InterveningVariablesPeoplevarywidelyintheirtoleranceofshiftwork,accordingtoHärmä(1993),whichmaybeexplainedbytheinfluenceofmanyinterveningvariables.Someindividualdifferenceswhichmaymodifythestrainofshiftworkersare:differencesinthephaseandamplitudeofthecircadiancycle,age,gender,pregnancy,physicalfitnessandflexibilityinsleepinghabits,andtheabilitytoovercomedrowsiness,asillustratedbyfigure43.1.Figure43.1ModelofstressandstrainofshiftworkersAlthoughsomeauthorsfoundacorrelationbetweenalargeramplitudeofcircadianrhythmsandfewermedicalcomplaints(Andlaueretal.1979;Reinbergetal.1988;Costaetal.1989;KnauthandHärmä1992),othershavefoundthatitdoesnotpredictadjustmenttoshiftwork(Costaetal.1989;MinorsandWaterhouse1981)evenafterthreeyearsofwork(Vidaceketal.1987).Thereappeartobetwomaindimensionsofpersonalityrelatedtothecircadianphase:“morningness”/“eveningness”andintroversion/extroversion(Kerkhof1985).Morningness/eveningnesscanbeassessedbyquestionnaire(HorneandÖstberg1976;Folkardetal.1979;TorsvalandÅkerstedt1980;Moog1981)or...
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