Commonly known as the ‘body clock’, the circadian rhythm is an innate cyclical rhythm that regulates many bodily functions automatically throughout the day and does not require conscious control.
There are those that are apparent to us, such as the sleep-wake cycle and the digestive cycle, for which we feel sleepy or hungry when we reach a certain time of the day. There are also those that are not so obvious, such as core body temperature and the release of hormones into the bloodstream.
In human beings, this innate rhythm cycles between the duration of 24.2 to 24.9 hours, just slightly longer than a day. This could potentially create a messy situation where we could fall asleep or need to eat at very inconvenient timings, over a period of time.
Fortunately, this ‘clock’ is synchronised to the 24-hour day by environmental inputs, most importantly by sunlight, as well as by social rhythm, such as common meal times, work schedules and physical exercises.
Genetics largely influence the variations between individuals, hence there are people whom we recognise as ‘larks’ (preferring to sleep early in the night) and ‘night owls’ (ability to stay up late into the night).
Genetics also determines the ability of individuals to adapt to time cues in the daily cycle, and hence the ability to ’tune their clocks’.
With age, this innate rhythm can also change in its cycle length, commonly reflected through changes in sleep pattern as one grows older.