[Fatigue and sleep: the point of view of the chronobiologist]

Chrono-fatigue is often present in patients engaged in shift or night work, as well as in passengers travelling across many time zones (jet lag syndrome). Some disturbances of the circadian system are responsible for clinical syndromes characterized by chrono-fatigue, one of them being the sleep delayed phase insomnia. Circadian sleep disorders, either functional or structural, are characterized not only by disturbances in sleep architecture and/or sleep quality, but also by the inability to fall asleep and stay awake at desired times. Similarly, sleep and fatigue present in those circadian disorders typically exhibit a daily cyclic pattern. Night workers and shift workers do have chronic sleep deprivation and desynchronisation of their circadian system with the entraining environment. Those phenomena are responsible for severe vigilance problems, a decrease in work productivity and deleterious health side effects. Fatigue and sleep problems in circadian disorders can now benefit from treatments based on the programed use of synchronizers of the circadian clock, like chronotherapy, bright light and melatonin, combined with adequate sleep hygiene recommendations.