Light and melatonin treatment for shift work

Between 10 and 30% of workers carry out night work at least once a month and 12–13% are working on rotating or regular night shifts. These atypical work schedules cause irregular, fragmented sleep patterns, as well as alertness and performance impairments at night. Atypical work schedules result in circadian misalignment, a state of desynchronization between the endogenous circadian system and the environment. Working at night also produces a state of internal desynchronization between several levels of the circadian system. Circadian adaptation to a night-oriented schedule is a gradual process requiring extended, consistent, and regular exposure to the altered work-rest cycle. There is a high degree of variability in the capacity of individuals to adapt to night schedules and it is estimated only 1 out of 4 workers is able to do so without specific interventions to facilitate circadian phase shifts. As light is the must powerful synchronizer of human circadian rhythms, countermeasures based on strategic light-dark interventions can favor circadian adaptation to atypical work schedules or modulate the secretion of melatonin at night. Melatonin, a hormone secreted by the pineal gland at night can also be administered exogenously to promote sleep in the daytime and shift human circadian rhythms. Most of our knowledge on the resetting and melatonin suppressive effects of light are based on the study of non-shift working populations in highly controlled laboratory conditions. In the field, exogenous melatonin proved useful to extend daytime sleep, although, still today, its resetting effects of shift workers' circadian rhythms remain unclear. The aim of this article is to review the latest scientific evidence on the usefulness of strategic light-dark interventions and on the use of melatonin and melatonin agonists prior to daytime sleep periods in shift working populations.