The human sleep-wake cycle has a period of approximately 24 hours, as the term circadian (Latin circa + dies) implies. If all external time indicators are removed, the circadian rhythm persists but the times of waking and going to sleep become later each day. The circadian rhythm is thought to be regulated by the suprachiasmatic nucleus of the hypothalamus (p. 142). Retino-hypothalamic connections tie the circadian rhythm to environmental light conditions. There is also a retinal projection to the pineal gland; the melatonin produced there has a rhythm-shifting effect.
Not only sleeping and waking but also many other bodily functions, including cardiovascular and respiratory function, hormone secretion, mitosis rate, intracranial pressure, and atten-tiveness, follow a circadian pattern (chronobi-^ ology). Circadian variation in performance is im-^ portant in the workplace and elsewhere. Some diseases are associated with certain times of the day (chronopathology)-certain types of epileptic seizures, asthma, cluster headache, gastro-esophageal reflux disease, myocardial infarction, vertricular tachycardia.
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