work, particularly work including night shifts, is the most widely studied condition, as it may interfere at several levels with human homeostasis and well-being.
At the biological level, the perturbation and, sometimes, the inversion of the sleep/wake cycle, connected with the modified activity/rest pattern, is a significant stress for the endogenous regulation of the “circadian” (of about 24 hours) rhythms of biological functions, which are driven by the body clock located in the suprachiasmatic nuclei of the encephalon and synchronised by environmental cues (the light/dark cycle in particular) through non-vision-related photic stimuli from retinal ganglion cells with high sensitivity to light [3-5].
Staying awake at night and trying to sleep during the day is not a physiological condition for diurnal creatures such as humans, who are hence forced to adjust their psycho-physio- logical state by a phase shift of the daily fluctuation of biologi- cal functions, which are normally activated during the day and depressed during the night. This phase shift occurs at a speed of about one hour per day and can widely vary according to the duration and extension of night duties along the shift schedule.
Workers involved in rotating shift work (the large major- ity) are subjected to a continuous stress to adjust as quickly as possible to the variable duty periods, which is partially and in- variably frustrated by the continuous changeovers, whereas per- manent night workers may adjust almost completely provided that they continue to maintain their inverted sleep/wake cycle also on their days-off [6].