SHIFT WORK DISORDER - myCME.com

Shift-work disorderFigure 3 Blood plasma levels of melatonin inindividual shift workers and control subjectsBlood melatonin levelsShift workersControls1234567891234561200 1800 2400 0600 1200Clock hourLines indicate nocturnal melatonin elevation (onset, offset, and duration) anddiamonds represent the peak of the rhythm (acrophase) in individuals.Reprinted with the permission of the American Physiological Society fromthe American Journal of Physiology: Regulatory, Integrative and ComparativePhysiology by Roden M, et al, 265, 1993. Permission conveyed through theCopyright Clearance Center, Inc. 47internal circadian rhythms to the timing of their newsleep/wake schedule. 40Accumulated sleep loss over successive nights asa result of shift work creates a growing sleep debt thatincreases the homeostatic sleep drive. 41 Over a seriesof night shifts, the natural circadian drive for sleep duringthe night interacts with this increasing sleep debt(FIGURE 2B), resulting in further exacerbation of excessivesleepiness, impaired work performance, and increasedrisk of accidents in individuals with SWD. 19Thus, both sleep loss as well as circadian pressure forsleep independently contribute to excessive sleepinessin patients with SWD.Although a change in sleep/wake relative to circadiantiming can trigger SWD, not all shift workers developthis CRSD. The high degree of variation betweenindividuals in terms of the severity of symptoms associatedwith shift work is a complex issue that has not yetbeen fully elucidated. However, it seems likely that thereare a number of innate factors that may increase an individual’ssusceptibility to SWD, including vulnerabilityto insomnia, sensitivity to sleep loss, or variation withinthe circadian system (TABLE 2). 15,16,42-46Studies of melatonin rhythms in night-shift workershave shown that many workers do not completely adapttheir circadian rhythms to their new pattern of sleep andwake 47,48 (FIGURE 3). This may be due to an inherent inabilityto adapt their circadian rhythms or due to behaviorsthat preclude adaptation. A recent study has shown thata significantly greater (P < .0001) number of shift-intolerantvs shift-tolerant workers have a circadian period thatis longer or shorter than 24 hours, indicative of circadiandesynchronization and an inability to adapt to their newwork schedule. 49 In addition, adaptation cannot occur innight-shift workers who persistently revert to a night-timesleep schedule on their days off and who, therefore, donot experience consistent circadian sleep/wake alignmentwith the light/dark cycle. Night-shift workers whodo not adapt to their new shift schedule have been reportedto experience reduced sleep during the daytime,putting them at increased risk of developing SWD comparedwith colleagues who demonstrated a rapid phaseshift to accommodate their new work schedule. 50The presence of noise in the home, poor sleep hygiene,and social obligations may make it difficult forsome shift workers to obtain a sufficient amount of sleep.In these instances, it may be that shift work is incompatiblewith the patient’s lifestyle, resulting in behaviorally inducedinsufficient sleep syndrome. In patients with SWD,however, insomnia and/or excessive sleepiness persistdespite attempts to fully accommodate the altered workschedule.SummaryThe mammalian circadian clock is complex and is responsiblefor ensuring the rhythmic nature of numerousbehaviors and processes. In recent years, there havebeen frequent and impressive advances in our understandingof the structure and properties of the mammaliancentral circadian oscillator—the SCN—and themolecular machinery that it controls.Of the 6 main CRSDs recognized by the InternationalClassification of Sleep Disorders (TABLE 1), 21 4 aredue to intrinsic problems with the circadian pacemaker,caused by damage to the SCN, maturational changes,lack of appropriate entrainment, or genetically inheritedtraits. The 2 remaining CRSDs—jet lag disorder andSWD—are triggered by behavioral changes, as they occuras a direct result of human activity, ie, long-distanceair travel in a short time and working outside usualhours, respectively. However, not everyone develops jetlag disorder or SWD under these conditions, and the interindividualvariation in susceptibility to intrinsic andextrinsic CRSDs is an area of ongoing research.In a round-the-clock, global society, shift-workingindividuals perform vital tasks, so it is imperative to findsimple ways to diagnose and treat SWD. The followingarticles discuss how this may be achieved. nS16 January 2010 / Vol 59, No 1 • Supplement to The Journal of Family Practice