Night

Night, or nighttime, is the period of darkness when the Sun is below the horizon. Daylight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of sunrise and sunset. Moonlight, airglow, starlight, and light pollution dimly illuminate night. The duration of day, night, and twilight varies depending on the time of year and the latitude. Night on other celestial bodies is affected by their rotation and orbital periods. The planets Mercury and Venus have much longer nights than Earth. On Venus, night lasts about 58 Earth days. The Moon's rotation is tidally locked, rotating so that one of the sides of the Moon always faces Earth. Nightfall across portions of the near side of the Moon results in lunar phases visible from Earth.

Organisms respond to the changes brought by nightfall: darkness, increased humidity, and lower temperatures. Their responses include direct reactions and adjustments to circadian rhythms governed by an internal biological clock. These circadian rhythms, regulated by exposure to light and darkness, affect an organism's behavior and physiology. Animals more active at night are called nocturnal and have adaptations for low light, including different forms of night vision and the heightening of other senses. Diurnal animals are active during the day and sleep at night; mammals, birds, and some others dream while asleep. Fungi respond directly to nightfall and increase their biomass. With some exceptions, fungi do not rely on a biological clock. Plants store energy produced through photosynthesis as starch granules to consume at night. Algae engage in a similar process, and cyanobacteria transition from photosynthesis to nitrogen fixation after sunset. In arid environments like deserts, plants evolved to be more active at night, with many gathering carbon dioxide overnight for daytime photosynthesis. Night-blooming cacti rely on nocturnal pollinators such as bats and moths for reproduction. Light pollution disrupts the patterns in ecosystems and is especially harmful to night-flying insects.

Historically, night has been a time of increased danger and insecurity. Many daytime social controls dissipated after sunset. Theft, fights, murders, taboo sexual activities, and accidental deaths all became more frequent due in part to reduced visibility. Despite a reduction in urban dangers, the majority of violent crime is still committed after dark. According to psychologists, the widespread fear of the dark and the night stems from these dangers. The fear remains common to the present day, especially among children.

Cultures have personified night through deities associated with some or all of these aspects of nighttime. The folklore of many cultures contains "creatures of the night", including werewolves, witches, ghosts, and goblins, reflecting societal fears and anxieties. The introduction of artificial lighting extended daytime activities. Major European cities hung lanterns housing candles and oil lamps in the 1600s. Nineteenth-century gas and electric lights created unprecedented illumination. The range of socially acceptable leisure activities expanded, and various industries introduced a night shift. Nightlife, encompassing bars, nightclubs, and cultural venues, has become a significant part of urban culture, contributing to social and political movements.

The word night is derived from the Old English niht. Both words are Germanic and cognates of the German nacht. The terms belong to a family of night words present in nearly all European languages, derived from an Indo-European word, reconstructed as *nekwt. The original root is thought most likely to be *nek-, a term relating to death. According to the nineteenth-century British philologist Walter William Skeat, this root meant 'to perish', 'to disappear', or 'to fail', with night being the point where light ceased and perished. More recently, Roland Pooth has argued for the root term to be understood to mean 'empty' or 'naked', with night being the point where the sky is naked and empty of light. As a result of this early origin, night shares its root with the Latin nox, the root of many English terms connected to the night, such as equinox and nocturnal.

Cognates of day are less widespread. Philologist Ernest Weekley attributed the many related night words to the early practice of measuring time in nights rather than days. The term fortnight, an Old English contraction of "fourteen nights", is a remnant of this ancient custom of measuring time in nights.

The letters "gh" were added to the word to represent the yogh character (Ȝ), unavailable on printing presses imported from continental Europe in the fifteenth and sixteenth centuries. As English speakers ceased to pronounce the yogh, the "gh" became silent. A similar process occurred in many English words, such as light.

A planet's rotation causes nighttime and daytime. When a place on Earth is pointed away from the Sun, that location experiences night. The Sun appears to set in the West and rise in the East due to Earth's rotation. Many celestial bodies, including the other planets in the solar system, have a form of night.