A mimosa kept in darkness proved in 1729 that living things keep time
In 1729 the French scientist Jean-Jacques d'Ortous de Mairan shut a Mimosa pudica plant away from daylight. Its leaves kept opening and closing on a roughly 24-hour schedule anyway. That simple test was the first designed to separate an internal clock from a response to the sun, and it launched the science of circadian rhythms.
Circadian, from Latin for 'about a day', describes internal rhythms of roughly 24 hours that persist without outside cues but can be reset by them. Light, temperature and other signals that reset the clock are called zeitgebers, German for 'time givers'. Such clocks appear in animals, plants, fungi and cyanobacteria, and evidence suggests they evolved separately in each group. A rhythm only counts as circadian if it keeps running in constant conditions; its free-running period tends to be slightly longer than a day in daytime-active animals and shorter in nocturnal ones.
People noticed the pattern long before they understood it. Theophrastus, in the 4th century BC, described a tree whose leaves closed at night and opened at sunrise, a report probably brought back by a ship's captain serving Alexander the Great; the tree was later identified as the tamarind. Chinese medical texts from around the 13th century linked treatments to times of day. In 1918 J. S. Szymanski showed animals keep 24-hour activity cycles without light or temperature cues, and in 1954 Colin Pittendrigh showed that fruit flies emerge from pupae on a circadian schedule that cooling delays but does not stop. Franz Halberg coined the word in 1959.
Genetics opened the clock's gears. In 1971 Ron Konopka and Seymour Benzer found the first clock mutation, in a fruit fly gene they named period. Work by Jeffrey Hall, Michael Rosbash and Michael Young uncovered the key genes and neurons of the fly clock and earned them the 2017 Nobel Prize in Physiology or Medicine. Joseph Takahashi found the first mammalian clock mutation in mice in 1994.
Humans have clock mutations too. In an extended family in Utah, affected members are extreme morning larks whose sleep and other rhythms run about four hours early. The cause is a single amino acid change in the PER2 protein. When human rhythms go awry clinically, the result is classed as a circadian rhythm sleep disorder.
Source: Circadian rhythm
