[Biologic rhythms. Circadian, ultradian and seasonal rhythms]

CIRCADIAN RHYTHMS: Our knowledge of the genetic and molecular mechanisms regulating the principal circadian clock located in the suprachiasmatic nuclei is progressing. The clock's intrinsic period varies from one species to another and to a lesser degree from one individual to another. In humans, the intrinsic period is slightly over 24 hours. The clock is capable of synchronizing itself to the surrounding environment by reacting to outside factors or zeitgebers (time-givers). Light-dark cycles are the main zeitgebers; meals, the social environment, and locomotor activity also affect the circadian clock. In addition, the circadian clock acts as an internal timer, providing the organism with a means of synchronizing the function of multiple biochemical and physiological systems. ULTRADIAN RHYTHMS: The frequency of ultradian rhythms varies considerably form one species to another and from one parameter to another. In humans, several functions oscillate at 60-120 minute intervals, rhythms which are sometimes superimposed on other functions oscillating at 3 to 5 minute intervals. SEASONAL RHYTHMS: Several mechanisms allow living organisms to adapt to seasonal variations in the environment. In certain species, reproduction functions are stimulated at appropriate moments in the yearly cycle, optimizing the newborn's chances of survival. Such seasonal variations are much less marked in humans.