Circadian rhythms are those daily cycles of physiology and behaviour that persist when organisms are isolated from the external world. They are expressed at all levels of life- from prokaryotic blue-green algae to higher plants and animals. Their biological role is to anticipate and thereby allow organisms to adapt to the solar day and night. In humans the cycle of sleep and wakefulness is the most obvious circadian rhythm, reflecting a profound alternation of brain state. Disruption of our circadian programme through shift work, old age and neurological disease is a significant and growing cause of chronic illness.
The principal circadian pacemaker is located in the suprachiasmatic nuclei of the hypothalamus (SCN), where individual neurons can operate as self-sustained circadian clocks.