Circadian Misalignment

When the timing of sleep is displaced, such as in shift work, the normal temporal relationship between the sleep-wake cycle and the endogenous pacemaker is perturbed. As physiology does not align with the inversed sleep-wake schedule, day sleep following a night shift, takes place during the period of maximum alertness, where sleep is difficult to maintain. This misalignment is believed to be the primary cause underlying shift workers’ sleep problems. Accordingly, the International Classification of Sleep Disorder (ICSD-2) has identified disordered sleep due to shift work as one of six types of distinct circadian rhythm sleep disorders (CRSD). The essential feature of CRSD is described as “a persistent and recurrent pattern of sleep disturbance due primarily to alterations in the circadian timekeeping system or a misalignment between the endogenous circadian rhythms and the exogenous factors that affect the timing or duration of sleep” (American Academy of Sleep Medicine, 2005).

Studies have shown that the duration and quality of sleep depends on the degree of misalignment between the sleep-wake cycle and the circadian clock, with shorter sleep

Benhaberou-Brun, & Paquet, 2001; Hennig, Kieferdorf, Moritz, Huwe, & Netter, 1998; Koller et al., 1994; Quera-Salva et al., 1997; Roden, Koller, Pirich, Vierhapper, & Waldhauser, 1993; Strogatz, Kronauer, & Czeisler, 1986). Experimental displacement of sleep has shown that sleep maintenance becomes increasingly difficult as sleep onset is postponed to early morning hours (Akerstedt & Gillberg, 1981). A shortening in sleep length can also be observed when having to wake-up very early in the morning, such as demanded on morning shifts. In order to get sufficient sleep on morning shifts, workers would have to go to sleep during the wake maintenance zone, where sleep is difficult to initiate. The earlier the need to rise, the more sleep is lost due to the difficulty to initiate sleep early enough in the evening in order to compensate for the need to get-up early (Akerstedt, 2003; Ingre, Kecklund, Akerstedt, Söderström, & Kecklund, 2008; Kecklund et al., 1997).

Such a misalignment does not only impair shift workers’ sleep but also their waking state (Dijk et al., 1992; Jewett & Kronauer, 1999). During morning and night shifts, work has to be accomplished at the circadian nadir of alertness --explaining the high sleepiness and fatigue commonly experienced by shift workers. Experiments using scheduled bright light exposure have shown improved subjective fatigue, performance, and mood as a function of circadian alignment (e.g. melatonin, cortisol and temperature) to the sleep-wake cycle (Baehr, Fogg, & Eastman, 1999; Campbell et al., 1995; Czeisler et al., 1990; Eastman, Boulos et al., 1995; Eastman, Hoese, Youngstedt, & Liu, 1995; Eastman, Liu, & Fogg, 1995; Eastman, Stewart, Mahoney, Liu, & Fogg, 1994; Martin & Eastman, 1998).

The night shift also imposes stress on the homeostatic process, as the time lapsed since waking is considerably longer than on evening and morning shifts. The effect of prior time awake is less established in shift work but has been clearly demonstrated in constant routine experiments (Dijk et al., 1992). This particularly applies to the first night shift

(Akerstedt, 2003), where time awake at shift end extends to 20-22 hours in comparison to only nine hours in day workers. The generally reduced sleep obtained prior to night shifts further increases sleepiness, with sleepiness scores accumulating over successive night shifts. A week of 4.5 hours of daily sleep has been shown to yield sleepiness scores comparable to total sleep deprivation (Carskadon & Dement, 1981).

1.3.1.1 Implications to health

A misalignment between the sleep-wake cycle and the endogenous clock causes physiological activation and hormone secretion at atypical times during the sleep-wake cycle, potentially disturbing the proper functioning of various bodily functions. The increased health problems of shift workers could also be a direct consequence of an internal misalignment between different physiological and hormonal rhythms. The digestive complaints of shift workers for example have been explained on the basis of irregular food intake as well as eating at inappropriate circadian times, during which the alimentary tract is not set to digest food properly (Folkard, Minors, & Waterhouse, 1985; Olson, 1984). Also melatonin and cortisol continue to fluctuate in a diurnal fashion, even after a series of consecutive inversed sleep-wake cycles, with cortisol peaking in the early morning hours and melatonin at night (Boivin & James, 2005; James, Walker, & Boivin, 2004; Sack, Blood et al., 1992; Weibel & Brandenberger, 1998). This causes a misalignment in the dose relationship between different hormones as well as a potential alteration in the normal dosage of secretion. Cortisol for example, has been shown to fluctuate with a higher amplitude in night workers than in day workers (James et al., 2004). Reinberg and colleagues found a positive association between the degree of internal de-synchronization

disturbed sleep, fatigue at work and negative mood (Reinberg et al., 1984; Reinberg & Ashkenazi, 2008; Reinberg et al., 1988).

Other studies, however, have failed to find this and some even suggest the opposite. A study by Roden and colleagues (1993) for example, found a greater degree of satisfaction in night workers whose circadian rhythms did not align with the inversed sleep-wake cycle but kept fluctuating on a diurnal phase. Similarly, Costa and colleagues found a positive association between digestive problems and degree of phase shifting in night workers (Costa, Lievore, Casaletti, Gaffuri, & Folkard, 1989). As such, the negative effects of circadian adjustment to shift work on health may be a consequence of social jetlag, due to the demands of shifting between work and free days or between the different shifts (e.g. between morning, evening, and night shifts).

This would suggest that circadian adjustment to shift work has opposing effects on health --although being favourable for sleep (while working on night and morning shifts), circadian alignment to displaced sleep often involves large phase shifts, which may potentially negatively affect health. Consequently, some have proposed a compromised entrainment, in which circadian rhythms are only partially entrained to the displaced sleep- wake cycle (Burgess, Sharkey, & Eastman, 2002; Eastman & Martin, 1999; Smith, Fogg, & Eastman, 2009). For partial entrainment to night work, workers are instructed to adopt as late a sleep schedule as possible on their days off, while circadian adjustment to workdays would be produced by means of scheduled bright light exposure. A compromised circadian phase position to night work has been shown to lead to increased mood, fatigue, and performance in permanent night workers (Smith et al., 2009).