Circadian rhythm/diurnal rhythm is biological rhythm or cycle, which fluctuates in intervals approximately 25 hours in controlled environment. It is associated with sleep-wake cycle, chronobiology, and biological clock.
The circadian rhythm of sleep and wakefulness is persisting even when environmental cues, such as light and temperature. It has a period slightly longer than 24 hours.
Circadian rhythms are controlled by a biological clock and work on a daily time scale. These affect body temperature, alertness, appetite, hormone secretion etc. as well as sleep timing.
Due to the circadian clock, sleepiness does not continuously increase as time passes. A person's desire and ability to fall asleep is influenced by both the length of time since the person woke from an adequate sleep, and by internal circadian rhythms.
Thus, the body is ready for sleep and for wakefulness at different times of the day. Among people with healthy circadian clocks, there is a continuum of phonotype from “larks”, “morning people”, which prefer to sleep and wake early, to “owls”, “evening people” or “night people”, which prefer to sleep and wake at late times (46).
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