Circadian rhythm

A circadian rhythm is any biological process that rises and falls on a cycle of about 24 hours, such as the sleep-wake cycle, body temperature, appetite and the release of hormones like melatonin. Such rhythms occur in most living things, from cyanobacteria and plants to fruit flies and people. They are generated by internal biological clocks, which outside cues keep in step with the day.

At the molecular level, the clock is a feedback loop of genes and the proteins they encode. In fruit flies, the PER protein builds up, pairs with a partner protein called TIM at night, and the pair shuts down production of more PER until it breaks down and the cycle starts again. Work uncovering this loop won the 2017 Nobel Prize in Physiology or Medicine. In mammals, a master clock in the suprachiasmatic nucleus of the hypothalamus receives light signals from the eyes and keeps the clocks in other tissues synchronized.

Light and darkness are the strongest cues, though meals, activity and temperature also play a part. Jet lag and shift work pull rhythms out of line with the outside world, and long-term disruption is linked to sleep disorders, obesity, diabetes and mood disorders.