On the other hand, in the case of continuous shift sched- ules, it was found that sleepiness decreases passing from a back- ward- to a forward-rotating shift system, as there are longer rest intervals between shifts [12].
Therefore, the combination of circadian disruption and sleep deficit can be responsible for high levels of sleepiness and fatigue during work periods, with consequently higher prone- ness to performance impairment, thus inducing or favoring er- rors and accidents.
During a normal day, alertness is high in the morning and early afternoon, being sustained by the circadian activation of biological rhythms and by the restoration given by a normal nocturnal sleep. It progressively decreases during the late after- noon and night hours, and, conversely, sleepiness increases due to the circadian drop of most psychophysical functions and to the prolonging of the time awake. Obviously, it further increas- es in conditions of repeated and cumulative sleep deprivation, like for several consecutive night shifts [13].
It is not yet clear whether persistent sleep disorders may establish/arise after many years of shift work. This is probably due to the difficulty of following up large groups of shift work- ers for several years and to the process of self-selection that occurs in the meantime; some retrospective studies (38) seem to suggest a cumulative effect, probably in more vulnerable and older workers.