Night

Night or nighttime is the period of ambient darkness when the Sun is below the horizon.

The word can be used in a social sense as the time between bedtime and morning. In common communication, it is a farewell (sometimes lengthened to "good night"), mainly when someone is going to sleep or leaving.^[1]

Astronomical night is the period between astronomical dusk and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky. As seen from latitudes between about 48.56° and 65.73° north or south of the equator, complete darkness does not occur around the summer solstice because, although the Sun sets, it is never more than 18° below the horizon at lower culmination, −90° Sun angles occur at the Tropic of Cancer on the December solstice and Tropic of Capricorn on the June solstice, and at the equator on equinoxes. And as seen from latitudes greater than 72° north or south of the equator, complete darkness does not occur in both equinoxes because, although the Sun sets, it is never more than 18° below the horizon.

The opposite of night is day (or "daytime", to distinguish it from "day" referring to a 24-hour period time). Twilight is the period of night after sunset or before sunrise when the Sun still illuminates the sky when it is below the horizon. At any given time, one side of Earth is bathed in sunlight (the daytime), while the other side is in darkness caused by Earth blocking the sunlight. The central part of the shadow is called the umbra, where the night is darkest.

Natural illumination at night is still provided by a combination of moonlight, planetary light, starlight, zodiacal light, gegenschein, and airglow. In some circumstances, aurorae, lightning, and bioluminescence can provide some illumination. The glow provided by artificial lighting is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems.

Duration and geography[edit]

On Earth, an average night is shorter than daytime due to two factors. Firstly, the Sun's apparent disk is not a point, but has an angular diameter of about 32 arcminutes (32'). Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the Sun is below the horizon by about 34'. The combination of these two factors means that light reaches the ground when the center of the solar disk is below the horizon by about 50'. Without these effects, daytime and night would be the same length on both equinoxes, the moments when the Sun appears to contact the celestial equator. On the equinoxes, daytime actually lasts almost 14 minutes longer than night does at the equator, and even longer towards the poles.

The summer and winter solstices mark the shortest and longest nights, respectively. The closer a location is to either the North Pole or the South Pole, the wider the range of variation in the night's duration. Although daytime and night nearly equalize in length on the equinoxes, the ratio of night to day changes more rapidly at high latitudes than at low latitudes before and after an equinox. In the Northern Hemisphere, Denmark experiences shorter nights in June than India. In the Southern Hemisphere, Antarctica sees longer nights in June than Chile. Both hemispheres experience the same patterns of night length at the same latitudes, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other is experiencing short nights (summer).

In the region within either polar circle, the variation in daylight hours is so extreme that part of summer sees a period without night intervening between consecutive days, while part of winter sees a period without daytime intervening between consecutive nights.^[2]

Beyond Earth[edit]

The phenomenon of day and night is due to the rotation of a celestial body about its axis, creating an illusion of the sun rising and setting. Different bodies spin at very different rates, some much faster than Earth and others extremely slowly, leading to very long days and nights. The planet Venus rotates once every 224.7 days – by far the slowest rotation period of any of the major planets. In contrast, the gas giant Jupiter's sidereal day is only 9 hours and 56 minutes.^[3] The length of a planet's orbital period determines the length of its day-night cycle as well - Venus has a rotation period of 224.7 days, but a day-night cycle just 116.75 days long due to its retrograde rotation and orbital motion around the Sun.^[4] Mercury has the longest day-night cycle as a result of its 3:2 resonance between its orbital period and rotation period - this resonance gives it a day-night cycle that is 176 days long.^[5] A planet may experience large temperature variations between day and night, such as Mercury, the planet closest to the sun.^[6]

The day-night cycle is one consideration for planetary habitability or the possibility of extraterrestrial life on distant exoplanets.^[7] 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.^[8][9]

Biology[edit]

Living organisms react directly to the darkness of night.^[11] Light and darkness also affect circadian rhythms, the physical and mental changes that occur in a 24-hour cycle.^[12] This daily cycle is regulated by an internal "biological clock" that is adjusted by exposure to light.^[12] The length and timing of nighttime depend on location and time of year.^[13] Organisms that are more active at night possess adaptations to the night's dimmer light, increased humidity, and lower temperatures.^[14]

Animals[edit]

Animals that are active primarily at night are called nocturnal and usually possess adaptations for night vision.^[15] In vertebrates' eyes, two types of photoreceptor cells sense light.^[16] Cone cells sense color but are ineffective in low light; rod cells sense only brightness but remain effective in very dim light.^[17] The eyes of nocturnal animals have a greater percentage of rod cells.^[16] 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.^[18]

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.^[15] Nocturnal insects navigate using moonlight, lunar phases, infrared vision, the position of the stars, and the Earth's magnetic field.^[19] Artificial lighting disrupts the biorhythms of many animals.^[20] Night-flying insects that use the moon for navigation are especially vulnerable to disorientation from increasing levels of artificial lighting.^[21] Artificial lights attract many night-flying insects that die from exhaustion and nocturnal predators.^[22] Decreases in insect populations disrupt the overall ecosystem because their larvae are a key food source for smaller fish.^[23] 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".^[24]

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.^[25] 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.^[25] Bats emit a steady stream of clicks while hunting insects and home in on prey as thin as human hair.^[26]

People and other diurnal animals sleep primarily at night.^[27] Humans, other mammals, and birds experience multiple stages of sleep visible via electroencephalography.^[28] The stages of sleep are wakefulness, three stages of non-rapid eye movement sleep (NREM) including deep sleep, and rapid eye movement (REM) sleep.^[29] During REM sleep, dreams are more frequent and complex.^[30] Studies show that some reptiles may also experience REM sleep.^[31] During deep sleep, memories are consolidated into long-term memory.^[32] 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.^[33]

Compared to waking life, dreams are sparse with limited sensory detail. Dreams are hallucinatory or bizarre, and they often have a narrative structure.^[34] Many hypotheses exist to explain the function of dreams without a definitive answer.^[34] Dreams that cause distress nightmares. The word "night-mare" originally referred to nocturnal demons that were believed to assail sleeping dreamers, like the incubus (male) or succubus (female).^[35] 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.^[35]

Fungi[edit]

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.^[13] By night, fungi are more engaged in synthesizing cellular components and increasing their biomass.^[36] 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.^[37] Few species of fungi have true circadian rhythms.^[13] A notable exception is Neurospora crassa, a bread mold, widely used to study biorhythms.^[38]

Plants[edit]

During the day, plants engage in photosynthesis and release oxygen. By night, plants engage in respiration, consuming oxygen and releasing carbon dioxide.^[39] Plants can draw up more water after sunset, which facilitates new leaf growth.^[40] As plants cannot create energy through photosynthesis after sunset, they use energy stored in the plant, typically as starch granules.^[41] Plants use this stored energy at a steady rate, depleting their reserves almost right at dawn.^[41] Plants will adjust their rate of consumption to match the expected time until sunrise. This avoids prematurely running out of starch reserves,^[41] and it allows the plant to adjust for longer nights in the winter.^[42] If a plant is subjected to artificially early darkness, it will ration its energy consumption to last until dawn.^[42]

Succulent plants, including cacti, have adapted to the limited water availability in arid environments like deserts.^[43] The stomata of cacti do not open until night.^[44] 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).^[44][45] Cacti and most night-blooming plants use CAM to store up 99% of the carbon dioxide they use in daily photosynthesis.^[46][47] Ceroid cacti often have flowers that bloom at night and fade before sunrise.^[48] As few bees are nocturnal, night-flowering plants rely on other pollinators including moths, beetles, and bats.^[49] These flowers rely more on the pollinators' sense of smell with strong perfumes to attract moths and foul-smelling odors to attract bats.^[50]

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.^[51][52] Cyanobacteria, also known as blue algae, switch from photosynthesis to nitrogen fixation after sunset,^[53] and they absorb DNA at a much higher rate.^[54]

Effects on life[edit]

Social[edit]

The first constant electric light was demonstrated in 1835.^[55] As artificial lighting has improved, especially after the Industrial Revolution, nighttime activity has increased and become a significant part of the economy in most places. Many establishments, such as nightclubs, bars, convenience stores, fast-food restaurants, gas stations, distribution facilities, and police stations now operate 24 hours a day or stay open as late as 1 or 2 a.m. Even without artificial light, moonlight sometimes makes it possible to travel or work outdoors at night.

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.^[56] It includes pubs, bars, nightclubs, parties, live music, concerts, cabarets, theatre, cinemas, and shows. These venues often require a cover charge for admission. Nightlife entertainment is often more adult-oriented than daytime entertainment.

Cultural and psychological[edit]

Night is often associated with danger and evil, because of the psychological connection of night's all-encompassing darkness to the fear of the unknown and darkness's hindrance of a major sensory system (the sense of sight). Nighttime is naturally associated with vulnerability and danger for human physical survival. Criminals, animals, and other potential dangers can be concealed by darkness. Midnight has a particular importance in human imagination and culture.

Upper Paleolithic art was found to show (by André Leroi-Gourhan) a pattern of choices where the portrayal of animals that were experienced as dangerous were located at a distance from the entrance of a cave dwelling at a number of different cave locations.^[58]

The belief in magic often includes the idea that magic and magicians are more powerful at night. Séances of spiritualism are usually conducted closer to midnight. Similarly, mythical and folkloric creatures such as vampires, ghosts and werewolves are described as more active at night. In almost all cultures, legendary stories warn of the night's dangers.

The cultural significance of the night in Islam differs from that in Western culture. The Quran was revealed during the Night of Power, the most significant night according to Islam. Muhammad made his famous journey from Mecca to Jerusalem and then to heaven in the night. Another prophet, Abraham, came to realize the supreme being in charge of the universe at night.

People who prefer nocturnal activity are called night owls.^[59]

See also[edit]

• Earth's shadow
• Night aviation regulations in the US
• Night sky
• Nocturne
• Olbers' paradox

Notes[edit]

References[edit]

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Further reading[edit]

Culture[edit]

• "International Night Studies Network". the CISAN-UNAM (México), the Center for Interdisciplinary Research on Montreal, McGill (Canada), the Institut de Géoarchitecture and the IDA-Brest (France). Archived from the original on 15 May 2021. Retrieved 23 May 2021.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
• Shaw, Robert (2 February 2018). The Nocturnal City (Ebook). Taylor & Francis. ISBN 9781317197225. Retrieved 23 May 2021 – via Google Books. ...This book looks at the relationship between night and society in contemporary cities..

External links[edit]

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