Earth spins around once in about a day, and most living things carry a clock for it. Most living things, including animals, plants and microorganisms, have body clocks called circadian rhythms that follow a roughly 24-hour cycle. Species, both nocturnal and diurnal, evolved with natural cycles of dark and light. The question for scientists was whether those clocks sit inside the organism or are simply a response to the Sun.
In 1729 Jean-Jacques d'Ortous de Mairan kept a heliotrope plant in constant darkness and saw its leaves still open and close each day, which was the first experiment to tell an inner clock from a response to sunlight. In 1823 Augustin de Candolle found that Mimosa leaves kept in constant conditions cycled every 22 to 23 hours, not 24. Yet for a long time scientists still credited light, temperature or magnetism, not an inner clock. In 1971 Ron Konopka and Seymour Benzer found the first body-clock mutation in fruit flies and named the gene period. Later work showed that the period gene sits at the centre of the fly's daily rhythm and that losing it stops that rhythm. The 2017 Nobel Prize in Physiology or Medicine went to Hall, Young and Rosbash for work on how a fruit fly protein helps control daily rhythms.
In humans the master clock is in the brain, in a spot called the SCN, and the SCN sits in the hypothalamus just above the optic chiasm. Special light-sensing cells containing melanopsin in the retina send it signals, and its cells keep each other in step, forming the body's master clock. Light, seen by the eyes, resets the body clock.
In the evening the brain's master clock tells the body to make more melatonin, a hormone that makes people sleepy. Traveling across time zones causes jet lag, because the body clock no longer matches the local day and night. After a long flight the body clock needs approximately one day to adjust for each hour of time-zone change, and flying west delays the clock while flying east advances it. Timing matters when treating jet lag, because melatonin works opposite to light: taken in the afternoon or early evening it moves the body clock earlier, and taken in the morning it moves it later.
People sleep best at night, when it is dark. Blue LED lighting suppresses melatonin production approximately five times more than orange-yellow sodium lights, and light from electronic devices at night can confuse body clocks. Humans need natural darkness for good sleep and health, and even dim light at night can disturb sleep and melatonin.
Many animals, like owls, are awake at night and rest in the day, and some hunt, find mates or hide from predators at night. Nearly half of Earth's species are nocturnal. Bright lights at night can confuse wildlife, such as migrating birds, sea turtle hatchlings and amphibians. About 83% of people, including 99% of Europeans and Americans, live under light-polluted skies more than 10% brighter than natural darkness, and 80% of North Americans cannot see the Milky Way. Night skies are getting brighter: globally, light pollution increased by at least 49% from 1992 to 2017, and by 16% between 2014 and 2022, though not evenly everywhere. Both figures are as of November 2026.
The planet is a clock too, and it is not perfectly steady either. The speed of Earth's rotation is not constant, so the day does not stay exactly the same length. In 1695 Edmond Halley noticed that the Moon seemed to be speeding up compared with ancient eclipse records. Later work showed that the real change was Earth's spin slowing down, a drag called tidal braking, and the tidal effects the Moon has on Earth's rotation are making Earth's days longer. Tides raised by the Moon and Sun add approximately 1.7 milliseconds to the length of a day each century.
Earth spins faster than the Moon orbits, so the tidal bulge runs ahead of the Moon and tugs it forward. The Moon's gravity pulls on Earth's tidal bulge in return, which gives the Moon energy and lets it spiral away while Earth's spin slows, and the Moon moves away from Earth approximately an inch (3.8 centimeters) a year. Scientists measure the Moon's recession by bouncing laser beams off it, and changes in the beam's round-trip time reveal how fast the Moon is moving away.
Scientists can read ancient day length from growth bands in corals and from layers in ancient rocks. Fossil shells show the day used to be shorter, since layers in a Late Cretaceous mollusc shell, 70 million years old, show 372 days in a year, so the day was approximately 23.5 hours long then. Rock and fossil records suggest the day was only approximately 21 to 22 hours long some 600 million years ago. Days now are a few hundredths of a second longer than they were around 1200 B.C.
In 1851 Léon Foucault hung a 28-kilogram bob on a 67-metre wire from the dome of the Panthéon in Paris, and its swing slowly turned, the first simple, direct proof of Earth's rotation. A Foucault pendulum at the North Pole keeps swinging in one plane while Earth turns beneath it, and the twisting effect is greatest at the poles, gets weaker toward lower latitudes and disappears at the equator.
The same rotation deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, and this is called the Coriolis effect. It is named for Gaspard-Gustave de Coriolis, whose 1835 paper was about the energy of machines with rotating parts, such as water wheels, not about weather. Because of the Coriolis effect, big low-pressure storms spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
A sundial and a clock rarely agree, since sundial time can run ahead by as much as 16 minutes 33 seconds, around 3 November, or behind by as much as 14 minutes 6 seconds, around 11 February. The sundial-clock gap, called the equation of time, has two causes. The first is that Earth's axis is tilted approximately 23.44 degrees, and the second is that its orbit is slightly oval (eccentricity approximately 0.0167), so the Sun's apparent daily motion is uneven. Day and night both come from Earth's spin, while seasons come from its tilt as it goes around the Sun, which takes 365.25 days, and the tilt points the same way throughout the year.
A hemisphere's summer solstice is its longest day, and when it is the summer solstice at one pole it is the winter solstice at the other. Polar night lasts from approximately 24 hours just inside the polar circles to 179 days at the poles. Because air bends sunlight over the horizon, polar day is longer than polar night, and the region of polar day is slightly bigger. In Svalbard, Norway, the Sun does not set from approximately 19 April to 23 August. The midnight sun reaches a little farther south than the Arctic Circle itself, occurring from approximately 65 degrees 44 minutes latitude to the pole. In the polar night, Longyearbyen has nautical polar twilight, enough light that the horizon is visible for navigation, from approximately 11 November until 30 January.
Before standard time, towns set their clocks by the Sun's path across their own meridian. Railroad travel made standard time necessary, and railroad leaders were asked to design a uniform time system. Railway companies in Great Britain adopted Greenwich Mean Time in 1847, and in the United States standard time began on Sunday, November 18, 1883. In 1884 delegates at the International Meridian Conference voted to make the Greenwich Meridian the world's Prime Meridian. UTC, today's world time standard, is based on atomic clocks that are more stable than the Earth's rotation, and it replaced GMT as the international civil time standard. UT1, the modern form of mean solar time, is based on the rotation of Earth, which is irregular. Earth's spin speeds up and slows down with weather, climate and geology, so leap seconds could not be scheduled far ahead. A standard body announced each one approximately six months early to keep UTC within 0.9 seconds of Earth-based time.
Other worlds turn on their own schedules, and a day can be very different from ours. One turn of Mars takes about 24.6 hours, close to a day on Earth, while Jupiter spins fastest of all the planets, so a day on Jupiter is under 10 hours. One day-night cycle on Mercury, from sunrise to sunrise, lasts approximately 176 Earth days, which is just over two Mercury years, and with no atmosphere its surface swings from about minus 180 to approximately 430 degrees Celsius. A world need not show one face to its star forever, because on an oval orbit the spin tends to lock to the orbital speed at the closest point to the star, which is why Mercury turns three times per two orbits. Venus spins backward, so one spin of 243 Earth days is longer than its year of approximately 224.7 Earth days, and on Venus the Sun would rise in the west. A solar day on Venus, from one noon to the next, lasts approximately 117 Earth days. Uranus is tipped approximately 98 degrees, spinning on its side, so its poles face the Sun for decades and then sit in a long, dark winter. Earth, by contrast, spins around once in about a day.
Words to know
- circadian: about a day
- hypothalamus: brain region
- melanopsin: light-sensing pigment
- melatonin: sleepy hormone
- eccentricity: how oval
- Coriolis: spin deflection
