Q.What are circadian rhythms?

Circadian rhythms are ~24-hour cycles of physiological and behavioural change (like sleep-wake, activity, hormone levels) generated by an internal biological clock and normally synchronized to the external day-night cycle.

Definition: Circadian rhythms (from Latin circa = 'about' and dies = 'day') are recurring biological cycles of behaviour or physiology that complete approximately once every 24 hours. They are governed by an internal, self-sustaining timekeeping mechanism often called the biological clock, but this internal clock is normally kept in step ('entrained') with the external environment by cues (zeitgebers) - chiefly the light-dark (day-night) cycle, and also temperature and feeding times.

Examples in animals:

• The human sleep-wake cycle, and daily fluctuations in body temperature and hormone levels (e.g., cortisol, melatonin).
• Daily activity/rest patterns of nocturnal and diurnal animals.
• Timing of migratory behaviour and feeding activity in birds and other animals is influenced by circadian and related seasonal (circannual) rhythms. …

A circadian rhythm is a roughly 24-hour cycle in an organism's physiology or behaviour, controlled by an internal 'biological clock' and reset daily by environmental cues, chiefly light.

The word 'circadian' comes from the Latin circa diem, meaning 'about a day'. Circadian rhythms are rhythmic biological processes — physiological, hormonal, or behavioural — that recur with a period of approximately 24 hours, even when an organism is kept under constant conditions (e.g. constant darkness), showing that the rhythm is internally generated rather than a passive response to day and night.

Key features:

• Generated by an internal 'biological clock' (in mammals, centred in the suprachiasmatic nucleus of the hypothalamus), which keeps time endogenously.
• Normally entrained (synchronised) to the actual 24-hour solar day by external cues called zeitgebers ('time-givers'), the most important of which is the light-dark cycle. …