In humans, circadian clocks affect many aspects of physiology: sleep-wake time, cardiac function (heartbeat and blood pressure), renal secretion, virtually all aspects of digestion from gastric emptying to detoxication, body temperature, and the levels of many hormones in the blood (Lavery and Schibler, 1999). In most tissues, about ten percent of genes exhibit a circadian pattern of expression (Akhtar et al., 2002; Panda et al., 2002; Storch et al., 2002). The period length of this clock, the time it takes for one cycle, is only approximately 24 hours. In constant darkness, the circadian clock will direct sleep-wake cycles and many other physiological processes according to its intrinsic period length, which may be longer or shorter than the solar day. In normal conditions, however, the clock is reset daily by light, and its period is instead believed to influence the relative phase of circadian physiology and activity patterns within each day. People with short periods are likely to be “morning types” or “larks”, and people with long periods are more often “late types” or “owls” (Duffy et al., 2001; Roenneberg et al., 2003).
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