## 2 Fatigue in Working Contexts
While circadian pacemaker contributes to the regulation of sleepiness within each day, sleep loss of as little as 2-3 h a night can contribute to sleepiness across days, in a cumulative manner [65; 66]. Figure 1: Circadian rhythm [32]. However, when sleep deprivation is continued for several days, the negative effects of sleep drive on alertness and performance continue to increase but the circadian process modulates the changes daily and can mitigate some effects of sleep loss during times of circadian peak [67]. This process predicts a nonlinearity of neurobehavioral performance, in contrast to the assumption that the longer one works the more fatigued one will become [68]. Indeed, several models developed in fatigue risk management are based on the amount of sleep obtained and on circadian phase as they dynamically interact over time modulating performance capability [49; 69]. These mathematical models are currently used to predict fatigue and to identify work schedules that pose a sleep deprivation risk and to estimate the magnitude of the risk [49]. However, the absence of feedback from actual values of neurobehavioral performance to improve the accuracy, is recognized as a limited [70]. Moreover, finding a model that predicts a momentary change of fatigue/sleepiness is very difficult. As recently suggested by [PERSON] and colleagues (2014) [68], integrated use of sleepiness prediction and detection technologies could be used to mitigate accidents and the risk of errors more effectively. As noted above the neurobehavioral effect of sleep loss and circadian periodicity follow a nonlinear time course within and between days, moreover there are individual differences in vulnerability to sleep loss and in circadian rhythm. Healthy adults show interindividual differences in circadian amplitude [71; 72], and circadian phase [71; 72; 73; 74], in part due to genetic influences [72]. Chronotype defined as morningness-eveningness type, is the most common interindividual variation in circadian rhythmicity. Morning and evening types differ in the circadian phase in their biological clocks [71; 73]. Generally, the morning type wakes up, feels better, and does well in the early hours, but feels sleepiness relatively early. Conversely, evening type prefers to wake up late in the morning and feel better at the end of the day (evening hours). To define chronotype, a self-report questionnaire such as the Horne-Ostberg morningness-eveningness questionnaire [75], and the Munich ChronoType Questionnaire, [76; 77], are used to differentiate timing of activities on workdays versus free days. Chronotypes are also affected by age [76; 78] particularly, adolescents tend towards evening types, they go to bed considerably later in the evening (particularly if they have access to television or live in latitudes where summer evenings can be very long) and so tend to be sleep-deprived on school days, whereas elderly become more of morning, while over 35 years old the most frequent chronotype is the morning one. What can be stated is that individuals are likely to have problems if they adopt an early lifestyle (retiring and rising earlier than average) but have a circadian system that tends to run later than average (for whatever reason), or vice versa. Such disparities can be important for those working shift systems [79]. Considering this, for example, morning types are expected to be less tolerant of night shift work than their evening counterparts [77]. Moreover, the number of older shift workers is growing in most developed countries due to the general aging of the working population, finally, employees with insufficient coping behaviors who are scheduled to work at times incompatible with their chronotype are more vulnerable to psychological problem [80].