Circadian, ultradian, and seasonal rhythms

Circadian rhythms: Our knowledge of the genetic and molecular mechanisms re gulating the principal circadian clock located in the suprachiasmatic nucle i is progressing. The clock's intrinsic period varies from one species to a nother and to a lesser degree from one individual to another. In humans, th e intrinsic period is slightly over 24 hours. The clock is capable of synch ronizing itself to the surrounding environment by reacting to outside facto rs or zeitgebers (time-givers). light dark cycles are the main zeitgebers; meals, the social environment, and locomotor activity also affect the circa dian clock In addition, the circadian clock acts as an internal timer, prov iding the organism with a means of synchronizing the function of multiple b iochemical and physiological systems. Ultradian rhythms: The frequency of ultradian rhythms varies considerably f orm one species to another and from one parameter to another. In humans, se veral functions oscillate at 60-120 minute intervals, rhythms which are som etimes superimposed on other functions oscillating at 3 to 5 minute interva ls. Seasonal rhythms: Several mechanisms allow living organisms to adapt to sea sonal variations in the environment. In certain species, reproduction funct ions are stimulated at appropriate moments in the yearly cycle, optimizing the newborn's chances of survival. Such seasonal variations are much less m arked in humans.