Understanding Circadian Rhythms

Every living organism on earth follows internal 24-hour cycles known as circadian rhythms. From a flower opening at dawn to a surge of human alertness in the mid-morning, these rhythms are deeply embedded in our biology. They regulate countless processes, from gene expression and metabolism to behaviour and sleep.

In mammals, the central pacemaker of this system is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN synchronises peripheral clocks that exist in nearly all tissues through neural, hormonal and behavioural signals. Light, especially short-wavelength, blue-white light detected by specialised retinal cells in the eye, serves as the primary environmental cue that entrains the SCN with the Earth’s natural day-night cycle.

When our exposure to light and darkness is disrupted, so too are our circadian rhythms. Spending too much time indoors, working night shifts, travelling across time zones or being exposed to artificial light at night can all interfere with this delicate system. Such disruption can lead to a range of metabolic, cognitive and physiological effects. Increasingly, research is showing how circadian misalignment contributes to disease risk, and how understanding temporal biology can guide both medical treatment and lifestyle choices.

The circadian clock is therefore not simply a passive timer but a fundamental regulatory system that keeps our internal physiology in tune with the external environment.

This short blog is inspired by the book On the Essential Principles and Practice of Circadian Biology: A Road Map by Roelof A. Hut and William J. Schwartz.