Circadian rhythms in general.

Circadian rhythms in humans are a mixture of endogenous and exogenous components that are derived from the body clock and the interaction between our environment and lifestyle. Inherently, the body clock tends to run slow (by solar time) with a period of about 25 hours. Under normal circumstances, however, zeitgebers adjust it to run with a period of exactly 24 hours. The important zeitgebers in humans appear to be a mixture of bright light and social factors. Recent evidence favors the hypothalamic suprachiasmatic nuclei as a site of the body clock in mammals, though other sites, including the pineal gland, might also play some role. Circadian rhythms not only enable us to adjust better to our rhythmic environment but also influence our responses to disease processes and drugs. Moreover, when our lifestyles are altered abruptly, our body clock is slow to adjust. The result is the symptoms associated with "jet lag" and the general malaise suffered by many nightworkers.