[Internal clock desynchronization, light and melatonin].

The internal clock is synchronized by environmental factors. In humans the main factors are the light-dark alternation, the sleep-wake cycle, and social life. Rhythm desynchronization occurs when the clock is no longer in phase (harmony) with the environment, resulting in a phase shift (phase advance or phase delay) which can produce fatigue, sleep disorders and mood disorders. Clock desynchronization is related to a a loss of adaptation between the clock and synchronizers, to an inability of the clock to be entrained, or to a dysfunction of the clock itself Shiftwork and nightwork, transmeridian flights, depressive states and other psychiatric disorders, as well as blindness, aging and intake in some medications and psychoactive agents like alcohol are among the numerous causes of rhythm desynchronization. Melatonin and light exposure are able to control and resynchronize the clock. The phase response curve (PRC) clearly demonstrates that light exposure and/or melatonin administration are able to shift (advance or delay, depending on their timing) and thereby reset the clock.