Night



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.

Astronomy
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.

Earth


The length of night on Earth varies depending on the time of year. Longer nights occur in winter, with the winter solstice being the longest. Nights are shorter in the summer, with the summer solstice being the shortest. Earth orbits the Sun on an axis tilted 23.44 degrees. Nights are longer when a hemisphere is tilted away from the Sun and shorter when a hemisphere is tilted toward the Sun. As a result, the longest night of the year for the Northern Hemisphere will be the shortest night of the year for the Southern Hemisphere.

Night's duration varies least near the equator. The difference between the shortest and longest night increases approaching the poles. At the equator, night lasts roughly 12 hours throughout the year. The tropics have little difference in the length of day and night. At the 45th parallel, the longest winter night is roughly twice as long as the shortest summer night. Within the polar circles, night will last the full 24 hours of the winter solstice. The length of this polar night increases closer to the poles. Utqiagvik, Alaska, the northernmost point in the United States, experiences 65 days of polar night. At the pole itself, polar night lasts 179 days from September to March.



Over a year, there is more daytime than nighttime because of the Sun's size and atmospheric refraction. The Sun is not a single point. Viewed from Earth, the Sun ranges in angular diameter from 31 to 33 arcminutes. When the center of the Sun falls level with the western horizon, half of the Sun will still be visible during sunset. Likewise, by the time the center of the Sun rises to the eastern horizon, half of the Sun will already be visible during sunrise. This shortens night by about 3 minutes in temperate zones. Atmospheric refraction is a larger factor. Refraction bends sunlight over the horizon. On Earth, the Sun remains briefly visible after it has geometrically fallen below the horizon. This shortens night by about 6 minutes. Scattered, diffuse sunlight remains in the sky after sunset and into twilight.



There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon. "Civil" twilight occurs when the Sun is between 0 to 6 degrees below the horizon. Nearby planets like Venus and bright stars like Sirius are visible during this period. "Nautical" twilight continues until the Sun is 12 degrees below the horizon. During nautical twilight, the horizon is visible enough for navigation. "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon. Beyond 18 degrees, refracted sunlight is no longer visible. The period when the sun is 18 or more degrees below either horizon is called astronomical night.

Similar to the duration of night itself, the duration of twilight varies according to latitude. At the equator, day quickly transitions to night, while the transition can take weeks near the poles. The duration of twilight is longest at the summer solstice and shortest near the equinoxes. Moonlight, starlight, airglow, and light pollution create the skyglow that dimly illuminates nighttime. The amount of skyglow increases each year due to artificial lighting.

Other celestial bodies


Night exists on the other planets and moons in the solar system. The length of night is affected by the rotation period and orbital period of the celestial object. The lunar phases visible from Earth result from nightfall on the Moon. The Moon has longer nights than Earth, lasting about two weeks. This is half of the synodic lunar month, the time it takes the Moon to cycle through its phases. The Moon is tidally locked to Earth; it rotates so that one side of the Moon always faces the Earth. The side of the Moon facing away from Earth is called the far side of the Moon and the side facing Earth is called the near side of the Moon. During lunar night on the near side, Earth is 50 times brighter than a full moon. Because the Moon has no atmosphere, there is an abrupt transition from day to night without twilight.



Night varies from planet to planet within the Solar System. Mars's dusty atmosphere causes a lengthy twilight period. The refracted light ranges from purple to blue, often resulting in glowing noctilucent clouds. Venus and Mercury have long nights because of their slow rotational periods. The planet Venus rotates once every 243 Earth days. Because of its unusual retrograde rotation, nights last 116.75 Earth days. The dense greenhouse atmosphere on Venus keeps its surface hot enough to melt lead throughout the night. Its planetary wind system, driven by solar heat, reverses direction from day to night. Venus's winds flow from the equator to the poles on the day side and from the poles to the equator on the night side. On Mercury, the planet closest to the Sun, the temperature drops over 1000 °F after nightfall.

The day-night cycle is one consideration for planetary habitability or the possibility of extraterrestrial life on distant exoplanets. Some exoplanets, like those of TRAPPIST-1, are tidally locked. Tidally locked planets have equal rotation and orbital periods, so one side experiences constant day, and the other side constant night. In these situations, astrophysicists believe that life would most likely develop in the twilight zone between the day and night hemispheres.

Biology


Living organisms react directly to the darkness of night. Light and darkness also affect circadian rhythms, the physical and mental changes that occur in a 24-hour cycle. This daily cycle is regulated by an internal "biological clock" that is adjusted by exposure to light. The length and timing of nighttime depend on location and time of year. Organisms that are more active at night possess adaptations to the night's dimmer light, increased humidity, and lower temperatures.

Animals
Animals that are active primarily at night are called nocturnal and usually possess adaptations for night vision. In vertebrates' eyes, two types of photoreceptor cells sense light. Cone cells sense color but are ineffective in low light; rod cells sense only brightness but remain effective in very dim light. The eyes of nocturnal animals have a greater percentage of rod cells. In most mammals, rod cells contain densely packed DNA near the edge of the nucleus. For nocturnal mammals, this is reversed with the densely packed DNA in the center of the nucleus, which reduces the scattering of light. Some nocturnal animals have a mirror, the tapetum lucidum, behind the retina. This doubles the amount of light their eyes can process.



The compound eyes of insects can see at even lower levels of light. For example, the elephant hawk moth can see in color, including ultraviolet, with only starlight. Nocturnal insects navigate using moonlight, lunar phases, infrared vision, the position of the stars, and the Earth's magnetic field. Artificial lighting disrupts the biorhythms of many animals. Night-flying insects that use the moon for navigation are especially vulnerable to disorientation from increasing levels of artificial lighting. Artificial lights attract many night-flying insects that die from exhaustion and nocturnal predators. Decreases in insect populations disrupt the overall ecosystem because their larvae are a key food source for smaller fish. Dark-sky advocate Paul Bogard described the unnatural migration of night-flying insects from the unlit Nevada desert into Las Vegas as "like sparkling confetti floating in the beam's white column".



Some nocturnal animals have developed other senses to compensate for limited light. Many snakes have a pit organ that senses infrared light and enables them to detect heat. Nocturnal mice possess a vomeronasal organ that enhances their sense of smell. Bats heavily depend on echolocation. Echolocation allows an animal to navigate with their sense of hearing by emitting sounds and listening for the time it takes them to bounce back. Bats emit a steady stream of clicks while hunting insects and home in on prey as thin as human hair.

People and other diurnal animals sleep primarily at night. Humans, other mammals, and birds experience multiple stages of sleep visible via electroencephalography. The stages of sleep are wakefulness, three stages of non-rapid eye movement sleep (NREM) including deep sleep, and rapid eye movement (REM) sleep. During REM sleep, dreams are more frequent and complex. Studies show that some reptiles may also experience REM sleep. During deep sleep, memories are consolidated into long-term memory. Invertebrates most likely experience a form of sleep as well. Studies on bees, which have complex but unrelated brain structures, have shown improvements in memory after sleep, similar to mammals.

Compared to waking life, dreams are sparse with limited sensory detail. Dreams are hallucinatory or bizarre, and they often have a narrative structure. Many hypotheses exist to explain the function of dreams without a definitive answer. Nightmares are dreams that cause distress. The word "night-mare" originally referred to nocturnal demons that were believed to assail sleeping dreamers, like the incubus (male) or succubus (female). It was believed that the demons could sit upon a dreamer's chest to suffocate a victim, as depicted in John Henry Fuseli's The Nightmare.

Fungi


Fungi can sense the presence and absence of light, and the nightly changes of most fungi growth and biological processes are direct responses to either darkness or falling temperatures. By night, fungi are more engaged in synthesizing cellular components and increasing their biomass. For example, fungi that preys on insects will infect the central nervous system of their prey, allowing the fungi to control the actions of the dying insect. During the late afternoon, the fungi will pilot their prey to higher elevation where wind currents can carry its spores further. The fungi will kill and digest the insect as night falls, extending fruiting bodies from the host's exoskeleton. Few species of fungi have true circadian rhythms. A notable exception is Neurospora crassa, a bread mold, widely used to study biorhythms.

Plants


During the day, plants engage in photosynthesis and release oxygen. By night, plants engage in respiration, consuming oxygen and releasing carbon dioxide. Plants can draw up more water after sunset, which facilitates new leaf growth. As plants cannot create energy through photosynthesis after sunset, they use energy stored in the plant, typically as starch granules. Plants use this stored energy at a steady rate, depleting their reserves almost right at dawn. Plants will adjust their rate of consumption to match the expected time until sunrise. This avoids prematurely running out of starch reserves, and it allows the plant to adjust for longer nights in the winter. If a plant is subjected to artificially early darkness, it will ration its energy consumption to last until dawn.

Succulent plants, including cacti, have adapted to the limited water availability in arid environments like deserts. The stomata of cacti do not open until night. When the temperature drops, the pores open to allow the cacti to store carbon dioxide for photosynthesis the next day, a process known as crassulacean acid metabolism (CAM). Cacti and most night-blooming plants use CAM to store up to 99% of the carbon dioxide they use in daily photosynthesis. Ceroid cacti often have flowers that bloom at night and fade before sunrise. As few bees are nocturnal, night-flowering plants rely on other pollinators including moths, beetles, and bats. These flowers rely more on the pollinators' sense of smell with strong perfumes to attract moths and foul-smelling odors to attract bats.

Eukaryotic and prokaryotic organisms that engage in photosynthesis are also affected by nightfall. Like plants, algae will switch to taking in oxygen and processing energy stored as starch. Cyanobacteria, also known as blue algae, switch from photosynthesis to nitrogen fixation after sunset, and they absorb DNA at a much higher rate.

History and technology


Before the industrial era, night was a time of heightened insecurity. Fear of the night was common but varied in intensity across cultures. Dangers increased due to lower visibility. Injuries and deaths were caused by drowning and falling into pits, ditches, and shafts. People were less able to evaluate others after dark. Due to nocturnal alcohol consumption and the anonymity of darkness, quarrels were more likely to escalate to violence. In medieval Stockholm, the majority of murders were committed while intoxicated. Crime and fear of crime increased at night. In pre-industrial Europe, ciminals disguised themselves with hats, facepaint, or cloaks. Thieves would trip pedestrians with ropes laid across streets and dismount horse riders using long poles extended from the roadside shadows. They used "dark lanterns" where light could be shined through a single side. Gangs were uncommon except for housebreaking. The increased humidity of night was deemed the result of vapors and fumes. Changes in the night sky were interpreted as significant omens.

Many daytime religious, governmental, and local social controls dissipated after nightfall. Fortified Christian communities announced the coming darkness with horns, church bells, or drums. This alerted residents&mdash;like peasants working the fields&mdash;to return home before the city gates shut. The English engaged in a daily process of "shutting in", where valuables were brought into homes before they were bolted, barred, locked, and shuttered. Many English and European towns attempted to impose curfews during the medieval period and gradually loosened the restrictions via exceptions. Prayer and folk magic were more common by night. Amulets were hung to ward off nightmares, spells were cast against thievery, and pig hearts were hung in chimneys to block demons from traveling down them. The common phrase "good night" has been shortened from "God give you a good night." In Ottoman Istanbul, the royal palaces shifted to projecting nocturnal power through large parties lit by lanterns, candles, and fireworks. Though alcohol was forbidden for Muslims, after dark, Turkish Muslims went to the bars and taverns beyond the Muslim areas.

The night has long been a time of increased sexual activity, especially in taboo forms such as premarital, extramarital, gay, and lesbian sex. In colonial New England courtship, young unmarried couples practiced bundling before marriage. The couples would lie down in the woman's bed, her family would wrap them tightly with blankets, and they would spend the night together this way. Some families took precautions to prevent unintended pregnancies, like sleeping in the same room, laying a large wooden board between the couple, or pulling a single stocking over both of their daughter's legs. Historian Roger Ekirch described pre-industrial night as a "sanctuary from ordinary existence."

Artificial lighting expanded the scope of acceptable work and leisure after dark. In the 1600s, the major European cities introduced streetlights. These were lit by lamplighters each evening outside of the summer months. Early street lights were metal and glass enclosures housing candles or oil lamps. They were suspended above streets or mounted on posts. The use of artificial lighting led to an increase in acceptable nightlife. In more rural areas, night remained a period of rest and nocturnal labor. Young adults, the urban poor, prostitutes, and thieves benefited from the anonymity of darkness and frequently smashed the new lanterns. Gas lighting was invented in the 1800s. A gas mantle was over ten times brighter than an oil lamp. Gas lighting was associated with the creation of regular police forces. In England, police departments were tasked with maintaining the gas lights, which became known as "police lamps". Daytime routines were further pushed back into the night by the electric light bulb&mdash;invented in the late 19th century&mdash;and the widespread usage of newer timekeeping devices like watches. Electric lights created night shifts for traditionally daytime fields, like India's cotton industry, and created opportunities for working adults to attend night school.

Before the widespread usage of artificial lighting, sleep was typically split into two major segments separated by about an hour of wakefulness. During this midnight period, people engaged in prayer, crimes, urination, sex, and, most commonly, reflection. Without exposure to artificial light, studies show that people revert to sleeping in two separate intervals.

Folklore and religion


Diverse cultures have made connections between the night sky and the afterlife. Many Native American peoples have described the Milky Way as a path where the deceased travel as stars. The Lakota term for the Milky Way is Wanáǧi Thacháŋku, or "Spirit's Road". In Mayan mythology, the Milky Way's dark band is the Road of Xibalba, the path to the underworld. Unrelated cultures share a myth of a star-covered sky goddess who arches over the planet after sunset, like Citlālicue, the Aztec personification of the Milky Way. The elongated Egyptian goddess Nut and N!adima from Botswana are said to consume the Sun at dusk. In the Ancient Egyptian religion, the Sun then travels through the netherworld inside Nut's body, where it is reborn at dawn.



Many cultures have personified the night. Ratri is the star-covered Hindu goddess of the night. In the Icelandic Prose Edda, night is embodied by Nótt. Ratri and Nött are goddesses of sleep and rest, but it's common for personifications to be associated with misfortune. In Aztec Mythology, Black Tezcatlipoca, the "Night Wind", was associated with obsidian and the nocturnal jaguar. In his "Precious Owl" manifestation, the Aztecs regarded Tezcatlipoca as the bringer of death and destruction. The Aztecs anticipated an unending night when the Tzitzimīmeh, skeletal female star deities, would descend to consume all humans. In classical mythology, the night goddess Nyx is the mother of Sleep, Death, Disease, Strife, and Doom. In Jewish culture and mysticism, the demon Lilith embodies the emotional reactions to darkness including terror, lust, and liberation.

Nighttime in the pre-industrial period, often called the "night season", was associated with darkness and uncertainty. Various cultures have regarded the night as a time when ghosts and other spirits are active on Earth. When Protestant theologians abandoned the concept of purgatory, many came to view reported ghost sightings as the result of demonic activity. In the sixteenth century, Swiss theologian Ludwig Lavater began attempting to explain reported spirits as mistakes, deceit, or the work of demons. The idea of night as a dangerous, dark, or haunted time persists in modern urban legends like the vanishing hitchhiker.

"Many times in the night season, there have been certain spirits heard softly going or spitting or groaning, who being asked what they were have made answer that they were the souls of this or that man and that they now endure extreme torments."

In folklore, nocturnal preternatural beings like goblins, fairies, werewolves, pucks, brownies, banshees, and boggarts have overlapping but non-synonymous definitions. The werewolf&mdash;and its francophone variations the loup-garou and rougarou&mdash;were believed to be people who transformed into beasts at night. In West Africa and among the African diaspora, there is a widespread tradition of a type of vampire who removes their human skin at night and travels as a blood-sucking ball of light. Variation includes the feu-follet, Surinamese asema, and Caribbean sukuyan, Ashanti obayifo, and Ghanaian asanbosam. The medieval fear of night-flying European witches was influenced by the Roman strix. The Romans described the strix as capable of changing between a beautiful woman and an owl-shaped monster. Common themes among these mythical nocturnal entities include hypersexuality, predation, shapeshifting, deception, mischief, and malice.

Nightlife


Nightlife is a collective term for entertainment that is available and generally more popular from the late evening into the early hours of the morning. It includes pubs, bars, nightclubs, parties, live music, concerts, cabarets, theatre, cinemas, and shows. Nightlife entertainment is often more adult-oriented than daytime entertainment. People who prefer to be active during the night-time are called night owls. Sociologists have argued that vibrant city nightlife scenes contribute to the development of culture and political movements. David Grazian cites as examples the development of beat poetry, musical styles including bebop, urban blues and early rock, and the importance of nightlife for the development of the gay rights movement in the United States kicked off by the riots at the Stonewall Inn nightclub in Greenwich Village, Lower Manhattan, New York City. Research conducted by Euromonitor International indicates a growing demand for unique, immersive nightlife experiences among millennials and Generation Zero.

Literature
In literature, night is often associated with mysterious, hidden, dangerous, and clandestine activities. Since the Age of Enlightenment, nocturnal settings have been a frequent place for passionate chaos as a counterbalance to the rationality present during the day. In Gothic fiction, this absence of rationality offered a space for lust and terror. Ottoman literature portrayed night as a time for forbidden or unrequited love. Night and day were long depicted as opposite conditions. The electric light, the industrial revolution, and shift work brought many aspects of daily life into the night. The author Charles Dickens lived in London during the time of gas lighting and compared the unstable separation between the waking and sleeping city, to the unstable separation he perceived between dream and delusion. Night in contemporary literature offers liminal settings, such as hospitals and gas stations, that contain some aspects of daily life.

"Night fell, while Helga Crane in the rushing swiftness of a roaring elevated train sat numb. It was as if all the bogies and goblins that had beset her unloved, unloving, and unhappy childhood had come to life with tenfold power to hurt and frighten."

Film and photography


Directly filming at night is rarely done. Film stocks and video cameras are much less sensitive to low light environments than the human eye. During the silent film era, night scenes were filmed during the day in black and white. The sections of the monochrome film reel with exterior night scenes were soaked in an acidic dye, that tinted the whole scene blue. "Day for night" is a set of cinematic techniques that simulate a night scene while filming in daylight. They include underexposing to the soften scene, using a graduated neutral-density filter to mute lighting, and setting up the artificial lighting to amplify shadows in the background. Lower budget films are more likely to use day for night shooting, larger budget films are more likely to film at night with artificial lighting. Cinematographers have used tinting, filters, color balance settings, and physical lights to color night scenes blue. In low light, people experience the Purkinje effect which causes reds to dim so that more blue is perceived. As light decreases towards total darkness, the human eye has more scotopic vision, relying more rod cells and less able to perceive color.



Night photography can capture the natural colors of night by increasing the exposure time, or length of time captured in the photography. Longer exposures open the possibility for photographers to use light painting to selectively illuminate a scene. Digital photography can also make use of high-ISO settings, which increase the sensitivity to light, to take shorter exposure shots. This makes it possible to capture moving subjects without turning their movements into a blur.