On the other hand, in the case of continuous shift schedules, it was found that sleepiness decreases passing from a backward 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 proneness to performance impairment, thus inducing or favoring errors 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.