Circadian rhythms are of endogenous origin, in humans as in all organisms. Under temporal isolation, i.e., after exclusion of all environmental time cues, circadian rhythmicity persists but with a period slightly deviating from 24 hours; in human, freerunning circadian rhythms always show periods close to 25 hours. In a minority of experiments, overt rhythms of different variables do not run in mutual synchrony but internally desynchronized in the steady state. This means that, indeed, most physiological rhythms, and particularly that of body temperature, hold a period close to 25 hours; it is mainly the sleep-wake rhythm (but also the overt rhythms of several more variables) which shows freerunning periods being considerably longer or shorter than 25 hours. This state of the rhythm is not concerned by the presence or absence of naps; rather, this state is characterized by a considerable stretching or compressing of the entire sleep-wake cycle. The period and other parameters of freerunning rhythms can be modified by continuously operating stimuli. Also the tendency toward the spontaneous occurrence of internal desynchronization does not depend only on personality data (e.g., neuroticism, or age) but also on the external conditions. Whereas constant light in the normal range of artificial illumination (intensities between 0 and 1500 lux) does not affect freerunning human circadian rhythms, the period is longer and the tendency toward internal desynchronization is higher under constant bright light (intensity greater than 3000 lux) than under constant light of normal intensity (or total darkness). This result has been confirmed with various physiological functions, e.g. the rhythms of deep body temperature and melatonin excretion. Social contacts (when subjects do not live singly isolated but in groups) or behavioral stress operates in the same direction as bright light. On the other hand, physical workload does not affect freerunning rhythms. Under natural conditions, the endogenously generated rhythms are synchronized to the 24-hour day. Under laboratory conditions (i.e., under temporal isolation), also artificial zeitgebers can be effective but only within limited ranges of periods; the width of such a range of entrainment is an indicator of the strength of the zeitgeber under consideration.(ABSTRACT TRUNCATED AT 400 WORDS)
