[Fundamentals of chronobiology: nyctohemeral rhythms].

Most biological activities fluctuate throughout the day and contribute to a better adaptation to the organism's daily activity. During the last 30 Years, chronobiology has aimed at studying these biological rhythms, explaining most of the biological mechanisms of i) the endogenous circadian rhythmicity, ii) the neurophysiological mechanisms of the photic system that allows its external resetting, and iii) the neuroendocrine mechanisms of internal rhythm synchronization. Moreover, the description of specific biological rhythm disorders and rhythm problems at the cellular and even the molecular level have prompted the emerging fields of chronopharmacology and chronotherapeutics.