Author(s): Svetlana Postnova 1 , 2 , 3 , * , Peter A. Robinson 1 , 2 , 3 , Dmitry D. Postnov 4
Shift work has become an essential part of our 24-h society. However, along with benefits of around-the-clock service, it leads to increased sleepiness of shift workers, which leads to accidents and work-related injuries. This can have detrimental consequences not only for the shift workers themselves, but also for people around them. This is especially the case in health care, transport, and public safety systems, where shift work is widespread, and lives depend on workers' performance -.
Increased sleepiness during the shifts is likely to be associated with misalignment of the circadian system and enforced sleep-wake schedules -. According to the two-process concept of Borbély  sleep-wake cycles are controlled by the circadian and homeostatic processes. The homeostatic process is responsible for the accumulation of sleep pressure during the time spent awake, and is hypothesized to be related to increase of somnogenic substances in the brain , or to synaptic plasticity -, but the precise mechanisms are still unclear. The circadian process controls the nearly 24-hour periodicity of the sleep-wake cycles, and is largely regulated by the master circadian clock in the suprachiasmatic nucleus of the hypothalamus (SCN) . The activity of the SCN is adjusted by a number of environmental inputs, with the strongest input being the light-dark cycle. Other inputs include meals, locomotion, and social interactions . In a person exposed to a natural light-dark cycle, the peak of the circadian activity appears during daytime and its minimum during the night. This rhythm is also reflected in fluctuations of the core body temperature (CBT), which demonstrates minimum during the night, usually 2-3 hours before awakening, and maximum during daytime. The timing of the CBT minimum is traditionally used as a marker of the circadian phase, as it is a reasonably precise and noninvasive measure .
Together, the homeostatic and circadian processes contribute to the level of the total sleep drive and determine the timing of sleep-wake transitions. Shift work leads to changes in the light exposure and sleep times, thereby affecting both the homeostatic and circadian processes. Ideally, the workers' circadian oscillators need to re-entrain in accord with the shift schedule; e.g., on the night shifts the maximum of circadian activity should appear during the night and minimum during the day, thus also allowing sufficient sleep time and recovery of the homeostatic sleep pressure. However, most of the time such re-entrainment does not happen even after many years of shift work, and the workers constantly perform in conditions of increased sleepiness and risk -.
The problem of adaptation to shift work had been intensively studied in the last decades. It has been demonstrated that increased light intensity during the shifts and darkness during the breaks can significantly improve adaptation to shifts and decrease sleepiness. Likewise, some shift schedules are expected to be easier to adapt to than others (for reviews see -). However, given...