Night Shifts: Chronotype and Social Jetlag
McKenna and Wilkes review on optimising sleep for night shifts is insightful and their advice helpful (1). However, in order to address the challenge posed by night shift work, it is necessary to appreciate the nature of the problem. This involves an appreciation of chronotype and the phenomenon social jetlag (2,3,4,5,6). Chronotype refers to an individual's circadian (daily) sleeping patterns when they have no commitments influencing waking and sleeping times (2,5,6). This is thus the behavioural and/or physiological manifestation of the intrinsic circadian clock. It is very well established that people fall into categories of chronotype (2,5,6). At the extremes are those whose circadian rhythm dictates that they go to bed early and rise early, even without the aid of an alarm and in the absence of any specific commitments. These are informally termed "larks"(2,5). At the other extreme are "owls", whose circadian clocks require that they go to sleep late and rise later (2,5).
These patterns are not a matter of choice but are deeply ingrained with even a strong genetic and heritable component (2,7,8). Work pattern, social, family responsibilities and lifestyle choices place constraints, altering the preferred sleeping pattern. You can determine your chronotype using the validated questionnaire (http://www.cet-surveys.com/index.php?sid=61524&newtest=Y). Some of us will be very polarised and at one extreme. Some of us will be more fluid in chronotype. Much of the advice regarding the best methods to address night shifts is contingent on chronotype. This relevant factor is not included in the constituent articles of the authors' review.
Social jetlag describes the disruption to the intrinsic circadian clock by societal commitments. It is quantitatively the difference, in hours, between the midpoint of sleep on a work/duties day and midpoint of sleep when an individual is divested of all commitments and responsibilities (2-5). So for example an "owl-ish" individual on a workday may sleep at 10pm and rise at 6am. On a free-day, however, may go to sleep at 1.00am and rise at 10.00am. Immediately this creates a social jetlag of 3.5 hours which accumulates through the working week (midpoint of sleep on word day 2am and midpoint of sleep on free-day 5.30am). Cumulative social jetlag has been linked to short-term declines in performance and long-term adverse health (2, 3, and 4). A lark may go to bed at 10am and rise at 6am irrespective of commitments and social calendar. Hence early chronotypes will not be able to sleep any longer on the day of the nightshift even when not roused by an alarm. However later chronotype will have little difficulty achieving this. It has been shown that those of a later chronotype tend to adapt better to working night shifts as they tend to accumulate a smaller social jetlag compared to early chronotypes, whose social jet lag can approach 11-12 hours every shift (8,9,10). This effect has been demonstrated in nurses working nightshifts (8).
One interesting study explored the effect of matching shifts to chronotype (10). In particular they avoided extreme shift-chronotype discordance. Hence early chronotypes avoided very late shift work and late chronotypes avoided very early shifts (10). The authors found that this significantly reduced social jetlag but more importantly improved well-being and sleep duration and quality between shifts (10). This is neither as feasible nor fair in the UK health service. Definitive solutions are difficult but an appreciation of the role chronotype and social jetlag help to understand the nature of the problem and reflect that night shifts pose a different circadian and indeed health challenge for different people.
(1) McKenna H, Wilkes M. Optimising sleep for night shifts. BMJ. 2018 Mar 1;360:j5637.
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(10) Vetter C, Fischer D, Matera JL, Roenneberg T.Aligning work and circadian time in shift workers improves sleep and reduces circadian disruption. Curr Biol. 2015 Mar 30;25(7):907-11
Competing interests: No competing interests