Night or nighttime is the period of darkness when the Sun is below the horizon. The opposite of nighttime is daytime. Sunlight illuminates one side of the Earth, leaving the other in darkness. 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 120 Earth days. The Moon's rotation is tidally locked, rotating so that the near side of the Moon always faces Earth. Nightfall across the near the side of the Moon results in the lunar phases visible from Earth.
Organisms respond to the changes brought by nightfall, including 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. 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.
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