circadian rhythm

Metabolic rhythm found in most organisms, which roughly coincides with the 24-hour day and is re-adjusted to precise day-length by light-sensitive mechanisms. Its most obvious manifestation is the regular cycle of sleeping and waking, but body temperature and the concentration of hormones that influence mood and behaviour also vary over the day. In humans, alteration of habits (such as rapid air travel round the world) may result in the circadian rhythm being out of phase with actual activity patterns, causing malaise until it has had time to adjust.

At the cellular level, circadian rhythms can be observed as a negative feedback loop in the activity of two genes, known as ‘period’ and ‘timeless’ in the fruit fly Drosophila. Similar cellular circadian clocks have been found in cells from many different body parts of animals, implying that there must be a central clock that controls the local ones.

In humans, the central time signals come from the pineal body, which produces and secretes the hormone melatonin at night. However, the central circadian clock is located in the suprachiasmatic nucleus (SCN) in the hypothalamus. Just how the light/dark changes reset our circadian rhythm each day was for a long time unexplained, but in the early 2000s several research groups found mounting evidence to suggest that a pigment known as melanopsin, which is found in a specific group of cells in the retina, the retinal ganglion cells, is the antenna which detects the light signal and communicates it to the SCN. Another group of light-sensitive proteins, the cryptochromes, may play an as yet unspecified role in light sensing.