Chapter 71 - Shift Work, Shift-Work Disorder, And Jet Lag

CHAPTER 71 Shift Work, Shift-Work Disorder, and Jet Lag 785Although some have speculated that permanent night workmay have benefits in terms of circadian adjustment to shiftwork relative to variable shift schedules and have advisedshift workers to stay on a night-shift schedule on days off,there is little support for this contention.6 Indeed, bothobjective and subjective measurements show that nightshifts result in greater loss of total sleep time than eveningand slow rotating-shift schedules.7-9 Sleep loss accumulatesand its impact grows over successive night shifts. Theresult is a buildup of homeostatic sleep debt combined withthe effects of circadian misalignment, with both havingserious implications for productivity and safety in shiftworkers. In fact, sleep loss alone has been shown to impairalertness and performance, including driving ability, asmuch as alcohol intake associated with a breath ethanolconcentration up to 0.19%.10 Not surprisingly, the nightshift produces the greatest degree of sleepiness relative todaytime work, evening shifts, and even rotating shifts, withthe sleepiness greatest during the early morning hoursclose to commute times.11,12Early Morning-Shift WorkersThe International Classification of Sleep Disorders (ICSD)classifies early morning shifts as those starting between 4am and 7 am.13 This is the most common alternate workshift with at least 18.1 million U.S. workers (12.4% ofthe workforce) falling into this category.5 Given thesestart times many early morning-shift workers wake before5 am. As a consequence, these workers are likely to beon the road at their nadir of circadian alertness and mayalso be particularly sleep deprived, owing to their earlytime of rising. This is consistent with the high rate ofexcessive sleepiness reported in this population.14 Usingobjective measures of sleep, Kecklund and colleaguesshowed that early morning-shift workers accrue significantly less sleep than those who work during the day;stage 2 and rapid eye movement (REM) sleep were particularly reduced.14 Indeed, their sleep disturbance is closeto that of permanent night workers.15 These factors,coupled with severe sleep inertia at that hour,16 suggestthat early morning-shift workers may have the highestrisk of all workers for automotive accidents. Furtherexamination of the prevalence of excessive sleepiness andresultant risk of accidents in this specific population isneeded.Evening/Afternoon-Shift WorkersEvening-shift workers with regular start times between 2pm and 6 pm make up 4.3% of all U.S. workers5 and canbe impaired in terms of social isolation and quality of life.17Although the literature clearly documents the presence ofsignificant morbidity in shift workers, not all shifts areassociated with these effects. In fact, the average eveningshift worker actually sleeps 7.6 hours/night,8 which islonger than most day workers (6.8-7.0 hours/night).12 Asnoted, the human circadian pacemaker has an intrinsicperiod that is on average slightly longer than 24 hours.18The resulting tendency to delay internal rhythms combined with schedules that allow later morning wakeuptimes may account for the increased total sleep time ofevening-shift workers. However, some evening-shiftworkers have shortened sleep times due to family obliga-tions that require earlier wakeup times on days off thatcould result in significant impairment over time.Rotating-Shift WorkersThe U.S. population is estimated to include 4 millionrotating-shift workers (approximately 2.7% of the totalworkforce),2 but nearly all shift workers could be considered to have rotating schedules because most (particularlymarried workers) revert to a normal pattern of nocturnalsleep during days off. Nonetheless, even on their daysoff, rotating-shift workers remain sleepier than daytimeworkers.19 In a meta-analysis of sleep patterns, workers onrotating shifts had nearly as much sleep reduction as permanent night workers relative to day workers.8Workers with regular rotating shift schedules face additional challenges related to the speed and direction of shiftrotations. Rapid shift rotations (e.g., multiple rotationwithin a week) are associated with reduced total sleep duration compared with slower rotations (e.g., at least 3 weeksper shift schedule).8 In terms of direction, both rapidclockwise and counterclockwise rotations negatively impacttotal sleep duration and increase circadian misalignment.20These effects are thought to be less severe for workersexperiencing a clockwise rotation because of the naturaltendency of the circadian clock to delay to a later time18and increased time between shifts. However, shift direction and duration may interact, with findings from at leastone controlled study showing that clockwise and counterclockwise rotations in rapidly rotating shift systems are notsignificantly different in terms of either total sleep duration or degree of excessive sleepiness.21 Some individualshave circadian clock periods that are shorter than 24 hours,and they would be expected to adapt more easily to counterclockwise shift rotations. Prior to a counterclockwiserotation, 80% to 90% of workers nap before the midnightshift, as opposed to only 40% to 60% before a clockwiserotation, which may help to ameliorate some of theexpected impairments in sleep and sleepiness during acounterclockwise rotation. This interpretation is also consistent with numerous studies demonstrating the beneficialeffects of napping among shift workers.22,23Circadian Misalignment and Effectsof Light ExposureHuman physiology is organized by the internal circadianclock such that sleep and its associated functions are promoted during the biological night, when levels of thehormone melatonin are high, and wakefulness and itsassociated functions are promoted during the biologicalday, when endogenous melatonin levels are low.24 Theinternal circadian clock in humans has a period that is onaverage slightly longer than 24 hours,18 and thus theaverage adult requires the phase and period of his or herinternal clock to be reset on a daily basis to remainentrained to the 24-hour day.25 Light is the dominantenvironmental time cue that entrains the human circadianclock to the 24-hour day, and the timing of light exposurewill determine whether the internal clock is phase delayedor advanced (Fig. 71-1).26 Circadian misalignment can becaused by schedules induced by shift work and jet travelthat rapidly alter exposure to environmental time cuesand require wakefulness during the biological night and

Maximum phaseadvance withmelatoninMaximum phaseadvance with lightPhase shiftDelaywestwardAdvanceeastward786 PART II / Section 9 Occupational Sleep MedicineMaximumphase delaywith light14 17 20 23 2Maximum phasedelay with melatonin58 11 14 17 20 23Approximate clock hour forsomeone with a bedtime at 24:00 hrFigure 71-1 Schematic representation of the phase responsecurves to 1 day of light exposure (6.7 hrs) (blue line) and 3 daysof 3 to 5 mg of exogenous melatonin administration (red line)when the circadian system is entrained to local environmentaltime. The circadian phase resetting response to light and melatonin depends on the internal biological time of exposure.Generally, bright light exposure before habitual bedtime andseveral hours thereafter will induce the largest westward phasedelays, whereas bright light exposure just before the habitualtime of awakening and several hours thereafter will induce thelargest eastward phase advances. The time at which phasedelays cross over to phase advances is on average about 2.5hours before the habitual time of awakening in young adultsand about 2 hours in older adults.153 Therefore, bright lightexposure too close to the crossover point may shift the circadian phase in a direction opposite to what is desired. Oppositeto the effects of light, ingestion of exogenous melatonin in thelate afternoon will induce the largest eastward phase advances,whereas melatonin ingestion shortly after the habitual time ofawakening and several hours thereafter will induce the largestwestward phase delays. The time at which melatonin-inducedphase delays change to phase advances is, on average, in theearly afternoon.103,104sleep during the biological day. The resulting circadianmisalignment associated with shift work and jet lag canproduce sig nificant morbidity associated with disturbedsleep, impaired alertness, as well as a direct effect of circadian misalignment.Morbidity Associated with Shift WorkSleepiness and InsomniaAmong the most common problems experienced by shiftworkers are excessive sleepiness and insomnia, resultingfrom imposition of a sleep–wake schedule that opposes thebody’s internal circadian clock.27,28 For example, measuresof sleepiness in anesthesia residents demonstrate impairedalertness after a single night shift similar to levels observedin sleep disorders.29 Chronic shift work tends to increasethe sleepiness, because shift workers may further deprivethemselves of sleep by reducing the amount of time spentin bed during the day.30 This is understandable, becauseeveryday domestic activities are likely to conflict with theopportunity for sleep. In addition, it is difficult to maintainsleep when the circadian clock is promoting wakefulness.27,28 Shift workers may also remain awake longer whenchanging from one shift schedule to another, producing afurther accumulation of sleep debt. Difficulty rapidly shifting the circadian clock combined with erratic exposure tolight in shift workers31 also places significant restrictionson adapting biological rhythms to shift-work schedules.Thus, in some workers, both sleep disturbance and sleepiness continue even after months or years of shift work.Reduced Alertness and AccidentsConverging evidence from controlled laboratory settings,large epidemiologic studies, and clinical samples has established an incontrovertible link between shift work andaccidents.32,33 In a classic study, Smith and colleagues demonstrated that working a night shift increased on-roadaccidents by 50%.34 In a recent study of a large sample ofnurses, 79.5% of those working the night shift reported atleast one drowsy driving incident, equal to an increasedodds ratio (OR) of 3.96 (95% confidence limit [CI] 3.244.84) relative to nurses working a day shift.35 Residentswho have frequent on-call schedules have 6.7 times the riskof motor vehicle accidents compared with those workingless-demanding call schedules.36 Others have shown thatrisk increases nearly 10% for every extended shift workedin a month.37Sleepiness-related impairment is not confined to motorvehicle accidents; for example, findings from studies ofmedical personnel have also demonstrated an increasednumber of accidents associated with the use of sharpinstruments and items (percutaneous injuries),38 medication and diagnostic errors,39 and increased patient deathrates associated with extended and unconventional shiftschedules.40 These findings are not surprising in light of astudy by Arnedt and colleagues showing that residents onheavy call rotations have driving impairments similar toresidents on light call rotations with a blood alcohol concentration of 0.05%.41 In industrial settings, the risk ofboth accidents and injuries increases by more than 30% onthe night shift; moreover, it increases over successive nightshifts and rises exponentially with successive hours on ashift.32Major catastrophes such as those at Three Mile Island,at Chernobyl, and from the Exxon Valdez oil spill have alloccurred during the night shift, drawing increased attention to both the risks and costs associated with shift workschedules.42,43 The cost of sleepiness-related accidents inthe United States is estimated at up to 40 billion per year,representing 24% of the total cost of traffic accidents inthe United States.44 These data suggest that the economicsavings associated with shift work for specific industriesshould be carefully weighed against the overall cost tosociety.Work Productivity and Quality of LifeThe negative impact of shift work is not limited to adverseevents. It also affects productivity. An association of shiftwork with reduced dexterity and efficiency,45 impairedthreat detection,46 and lower productivity47 has been demonstrated. Thus, overall worker productivity is significantly reduced during the night shift in a broad range ofoccupational settings.42 There is also evidence for increasedabsenteeism in night workers compared with day workers,particularly for those experiencing insomnia and/or excessive sleepiness.12

CHAPTER 71 Shift Work, Shift-Work Disorder, and Jet Lag 787The negative effects of shift work affect the familysystem as well as the individual’s quality of life.48 Studiesshow a 57% higher divorce rate,49 reduced job satisfaction,50 and reduced family and social interaction.12 Findings from a 5-year longitudinal study demonstrated arelationship between parental shift work and poor schoolperformance and behavioral problems in children 5 to12 years old after controlling for a number of demographic variables.51 The reader is also referred to moreextensive reviews on work-related and quality-of-lifevariables in shift workers.48Health Effects of Shift WorkA wealth of information documents the negative healtheffects associated with shift work. Notable are a 36% to60% increased risk of breast cancer in large prospectivestudies,52,53 a fourfold increased risk of duodenal ulcersverified by endoscopy,54 and increased cardiovascular morbidity and mortality,55-57 including atherosclerosis andmyocardial infarction.55 Poor eating habits58 and otheradverse health behaviors among shift workers may accountfor some of the increased morbidity. Moreover, findingsfrom one study demonstrated that a weekly 12-hour shiftin the light–dark cycle relative to endogenous circadianrhythms decreased survival by 11% in hamsters with cardiomyopathic heart disease.59 This finding suggests thatthese morbidities are not simply related to health habitsbut may be inherently related to misalignment of circadianrhythms and the sleep–wake cycle.With respect to the risk of breast cancer, investigatorshave suggested that reduced free radical scavenging due tosuppression of the hormone melatonin by nocturnal lightexposure, thereby redu

CHAPTER 71 Shift Work, Shift-Work Disorder, and Jet Lag. 785 tions that require earlier wakeup times on days o