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Owls are well known for being nocturnal, but some owls are active during the day. Northern Spotted Owl.USFWS.jpg
Owls are well known for being nocturnal, but some owls are active during the day.

Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.


Nocturnal creatures generally have highly developed senses of hearing, smell, and specially adapted eyesight. [1] Some animals, such as cats and ferrets, have eyes that can adapt to both low-level and bright day levels of illumination (see metaturnal). Others, such as bushbabies and (some) bats, can function only at night. Many nocturnal creatures including tarsiers and some owls have large eyes in comparison with their body size to compensate for the lower light levels at night. More specifically, they have been found to have a larger cornea relative to their eye size than diurnal creatures to increase their visual sensitivity: in the low-light conditions. [2] Nocturnality helps wasps, such as Apoica flavissima , avoid hunting in intense sunlight.

Diurnal animals, including squirrels and songbirds, are active during the daytime. Crepuscular species, such as rabbits, skunks, tigers, and hyenas, are often erroneously referred to as nocturnal. Cathemeral species, such as fossas and lions, are active both in the day and at night.


The Kiwi is a family of nocturnal birds endemic to New Zealand. Kiwi bird in Christchurch, New Zealand, 2002-01-01.jpg
The Kiwi is a family of nocturnal birds endemic to New Zealand.

While it is difficult to say which came first, nocturnality or diurnality, there is a leading hypothesis out in the evolutionary biology community. Known as the "bottleneck theory", it postulates that millions of years ago in the Mesozoic era, many ancestors of modern-day mammals evolved nocturnal characteristics in order to avoid contact with the numerous diurnal predators. A recent study attempts to answer the question as to why so many modern day mammals retain these nocturnal characteristics even though they are not active at night. The leading answer is that the high visual acuity that comes with diurnal characteristics isn't needed anymore due to the evolution of compensatory sensory systems, such as a heightened sense of smell and more astute auditory systems. [3] In a recent study, recently extinct elephant birds and modern day nocturnal kiwi bird skulls were examined to recreate their likely brain and skull formation. They indicated that olfactory bulbs were much larger in comparison to their optic lobes, indicating they both have a common ancestor who evolved to function as a nocturnal species, decreasing their eyesight in favor of a better sense of smell. [3] The anomaly to this theory were anthropoids, who appeared to have the most divergence from nocturnality than all organisms examined. While most mammals didn't exhibit the morphological characteristics expected of a nocturnal creature, reptiles and birds fit in perfectly. A larger cornea and pupil correlated well with whether these two classes of organisms were nocturnal or not. [2]


Resource competition

Honey Badger, a Nocturnal animal. Honey Badger (Mellivora capensis) (17425848662).jpg
Honey Badger, a Nocturnal animal.

Being active at night is a form of niche differentiation, where a species' niche is partitioned not by the amount of resources but by the amount of time (i.e. temporal division of the ecological niche). Hawks and owls can hunt the same field or meadow for the same rodents without conflict because hawks are diurnal and owls are nocturnal. This means they are not in competition for each other's prey.


Echolocating bats adjust their vocalizations to catch insects against a changing environmental background. Bat flying at night.png
Echolocating bats adjust their vocalizations to catch insects against a changing environmental background.

Nocturnality is a form of crypsis, an adaptation to avoid or enhance predation. One of the reasons that (cathemeral) lions prefer to hunt at night is that many of their prey species (zebra, antelope, impala, wildebeest, etc.) have poor night vision. Many species of small rodents, such as the Large Japanese Field Mouse, are active at night because most of the dozen or so birds of prey that hunt them are diurnal. There are many diurnal species that exhibit some nocturnal behaviors. For example, many seabirds and sea turtles only gather at breeding sites or colonies at night to reduce the risk of predation to themselves and/or their offspring. Nocturnal species take advantage of the night time to prey on species that are used to avoiding diurnal predators. Some nocturnal fish species will use the moonlight to prey on zooplankton species that come to the surface at night. [4] Some species have developed unique adaptations that allow them to hunt in the dark. Bats are famous for using echolocation to hunt down their prey, using sonar sounds to capture them in the dark.

Water conservation

Another reason for nocturnality is avoiding the heat of the day. This is especially true in arid biomes like deserts, where nocturnal behavior prevents creatures from losing precious water during the hot, dry daytime. This is an adaptation that enhances osmoregulation. [5] One of the reasons that (cathemeral) lions prefer to hunt at night is to conserve water.

Many plant species native to arid biomes have adapted so that their flowers only open at night when the sun's intense heat cannot wither and destroy their moist, delicate blossoms. These flowers are pollinated by bats, another creature of the night.

Climate-change and the change in global temperatures has led to an increasing amount of diurnal species to push their activity patterns closer towards crepuscular or fully nocturnal behavior. This adaptive measure allows species to avoid the heat of the day, without having to leave that particular habitat. [6]

Human disturbances

The exponential increase in human expansion and technological advances in the last few centuries has had a major effect on nocturnal animals, as well as diurnal species. The causes of these can be traced to distinct, sometimes overlapping areas: light pollution and spatial disturbance.

Light pollution

Light pollution on a ski slope in Finland gives the area a hazy, brightened sky. Light Pollution (5346483205).jpg
Light pollution on a ski slope in Finland gives the area a hazy, brightened sky.

Light pollution is a major issue for nocturnal species, and the impact continues to increase as electricity reaches parts of the world that previously had no access. [7] Species in the tropics are generally more affected by this due to the change in their relatively constant light patterns, but temperate species relying on day-night triggers for behavioral patterns are also affected as well. Many diurnal species see the benefit of a "longer day", allowing for a longer hunting period, which is detrminental to their nocturnal prey trying to avoid them. [4]


Light pollution can disorient species that are used to darkness, as their adaptive eyes are not as used to the artificial lighting. Insects are the most obvious example, who are attracted by the lighting and are usually killed by either the heat or electrical current. [8] Some species of frogs are blinded by the quick changes in light, while nocturnal migratory birds may be disoriented, causing them to lose direction, tire out, or be captured by predators. [4] Sea turtles are particularly affected by this, adding to a number of threats to the different endangered species. Adults are likely to stay away from artificially lit beaches that they might prefer to lay eggs on, as there is less cover against predators. [4] [8] Additionally, baby sea turtles that hatch from eggs on artificially lit beaches often get lost, heading towards the light sources as opposed to the ocean. [8]

Rhythmic behaviors

Rhythmic behaviors are affected by light pollution both seasonally and daily patterns. Migrating birds or mammals might have issues with the timing of their movement for example. [8] On a day-to-day basis, species can see significant changes in their internal temperatures, their general movement, feeding, and body mass. [9] These small scale changes can eventually lead to a population decline, as well as hurting local trophic levels and interconnecting species. [9] Some typically diurnal species have even become crepuscular or nocturnal as a result of light pollution and general human disturbance. [9]


There have been documented effects of light pollution on reproductive cycles and factors in different species. It can affect mate choice, migration to breeding grounds, and nest site selection. [4] In male green frogs, artificial light causes a decrease in mate calls, and continued to move around instead of waiting for a potential mate to arrive. [10] This hurts the overall fitness of the species, which is concerning considering the overall decrease in amphibian populations. [10]


Some nocturnal predator-prey relationships are interrupted by artificial lighting. Bats that are fast-moving are often at an advantage with insects being drawn to light; they are fast enough to escape any predators also attracted to the light, leaving slow-moving bats at a disadvantage. [4] Another example is harbor seals eating juvenile salmon that moved down a river lit by nearby artificial lighting. Once the lights were turned off, predation levels decreased. [4] Many diurnal prey species forced into being nocturnal are susceptible to nocturnal predators, and those species with poor nocturnal eyesight often bear the brunt of the cost. [9]

Spatial disturbance

The increasing amount of habitat destruction worldwide as a result of human expansion has given both advantages and disadvantages to different nocturnal animals. As a result of peak human activity in the daytime, more species are likely to be active at night in order to avoid the new disturbance in their habitat. [11] Carnivorous predators however are less timid of the disturbance, feeding on human waste and keeping a relatively similar spatial habitat as they did before. [11] In comparison, herbivorous prey tend to stay in areas where human disturbance is low, limiting both resources and their spatial habitat. This leads to an imbalance in favor of predators, who increase in population and come out more often at night. [11]

In captivity


In zoos, nocturnal animals are usually kept in special night-illumination enclosures to invert their normal sleep-wake cycle and to keep them active during the hours when visitors will be there to see them.


Hedgehogs are mostly nocturnal. European hedgehog (Erinaceus europaeus).jpg
Hedgehogs are mostly nocturnal.

Hedgehogs and sugar gliders are just two of the many nocturnal species kept as (exotic) pets. Cats have adapted to domestication so that each individual, whether stray alley cat or pampered housecat, can change their activity level at will, becoming nocturnal or diurnal in response to their environment or the routine of their owners. Cats normally demonstrate crepuscular behavior, bordering nocturnal, being most active in hunting and exploration at dusk and dawn. [12]

See also

Related Research Articles

Warm-blooded animal species that can maintain a body temperature higher than their environment

Warm-blooded are those animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The only known living homeotherms are birds and mammals, though ichthyosaurs, pterosaurs, plesiosaurs and non-avian dinosaurs are believed to have been homeotherms. Other species have various degrees of thermoregulation.

Night monkey genus of mammals

The night monkeys, also known as the owl monkeys or douroucoulis, are the members of the genus Aotus of New World monkeys. The only nocturnal monkeys, they are native to Panama and much of tropical South America. Night monkeys constitute one of the few monkey species that are affected by the often deadly human malaria protozoan Plasmodium falciparum, making them useful as non-human primate experimental subjects in malaria research.

Ecological niche The fit of a species living under specific environmental conditions.

In ecology, a niche is the match of a species to a specific environmental condition. It describes how an organism or population responds to the distribution of resources and competitors and how it in turn alters those same factors. "The type and number of variables comprising the dimensions of an environmental niche vary from one species to another [and] the relative importance of particular environmental variables for a species may vary according to the geographic and biotic contexts".

Crepuscular animal Animal primarily active during twilight

A crepuscular animal is one that is active primarily during the twilight period. This is distinguished from diurnal and nocturnal behavior, where an animal is active during the hours of daylight and of darkness, respectively. Some crepuscular animals may also be active by moonlight or during an overcast day. Matutinal animals are active only before sunrise, and vespertine only after sunset.

Endotherm organism that maintains its body at a metabolically favorable temperature

An endotherm is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat set free by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as shivering, and uncoupled oxidative metabolism such as within brown adipose tissue. Only birds and mammals are extant universally endothermic groups of animals. Certain lamnid sharks, tuna and billfishes are also endothermic.

Anti-predator adaptation Defensive feature of prey for selective advantage

Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avoiding detection, warding off attack, fighting back, or escaping when caught.

Cathemerality, sometimes called metaturnality, is the behaviour in which an organism has sporadic and irregular intervals of activity during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed. It has been defined as follows: "The activity of an organism may be regarded as cathemeral when it is distributed approximately evenly throughout the 24 h of the daily cycle, or when significant amounts of activity, particularly feeding and/or traveling, occur within both the light and dark portions of that cycle."

Crypsis ability of an organism to avoid observation, detection

In ecology, crypsis is the ability of an animal to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle and mimicry. Crypsis can involve visual, olfactory, or auditory concealment. When it is visual, the term cryptic coloration, effectively a synonym for animal camouflage, is sometimes used, but many different methods of camouflage are employed by animals.

Diurnality animal behavior characterized by activity during the day, with a period of sleeping, or other inactivity, at night

Diurnality is a form of plant or animal behavior characterized by activity during daytime, with a period of sleeping or other inactivity at night. The common adjective used for daytime activity is "diurnal". The timing of activity by an animal depends on a variety of environmental factors such as the temperature, the ability to gather food by sight, the risk of predation, and the time of year. Diurnality is a cycle of activity within a 24-hour period; cyclic activities called circadian rhythms are endogenous cycles not dependent on external cues or environmental factors. Animals active during twilight are crepuscular, those active during the night are nocturnal, and animals active at sporadic times during both night and day are cathemeral.

Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The migration occurs when organisms move up to the uppermost layer of the sea at night and return to the bottom of the daylight zone of the oceans or to the dense, bottom layer of lakes during the day. The word diel comes from the Latin dies day, and means a 24-hour period. In terms of biomass, it is the greatest migration in the world. It is not restricted to any one taxon as examples are known from crustaceans (copepods), molluscs (squid), and ray-finned fishes (trout). Various stimuli are responsible for this phenomenon, the most prominent being response to changes in light intensity, though evidence suggests that biological clocks are an underlying stimulus as well. The phenomenon may arise for a number of reasons, though it is most typically to access food and avoid predators. While this mass migration is generally nocturnal, with the animals ascending from the depths at nightfall and descending at sunrise, the timing can be altered in response to the different cues and stimuli that trigger it. Some unusual events impact vertical migration: DVM is absent during the midnight sun in Arctic regions and vertical migration can occur suddenly during a solar eclipse.

Sensory ecology is a relatively new field focusing on the information organisms obtain about their environment. It includes questions of what information is obtained, how it is obtained, and why the information is useful to the organism.

Ili pika species of mammal

The Ili pika is a species of mammal in the family Ochotonidae, endemic to northwest China. After its discovery in 1983, it was not documented again until 2014. Its population is declining, likely due to the effects of climate change. The Ili pika is currently considered to be endangered, with approximately fewer than 1,000 left.


Matutinal, matinal, and matutine are terms used in the life sciences to indicate something of, relating to, or occurring in the early morning. The term may describe crepuscular animals that are significantly active during the predawn or early morning hours. During the morning twilight period and shortly thereafter, these animals partake in important tasks, such as scanning for mates, mating, and foraging.

Sleep in non-human animals sleep in non-human beings

Sleep in non-human animals refers to a behavioral and physiological state characterized by altered consciousness, reduced responsiveness to external stimuli, and homeostatic regulation. Sleep is observed in mammals, birds, reptiles, amphibians, and some fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep at night for diurnal organisms and in the day for nocturnal organisms. Sleep patterns vary widely among species. It appears to be a requirement for all mammals and most other animals.

Bat Order of flying mammals

Bats are mammals of the order Chiroptera. With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more manoeuvrable than birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti's hog-nosed bat, which is 29–34 millimetres in length, 150 mm (6 in) across the wings and 2–2.6 g in mass. The largest bats are the flying foxes and the giant golden-crowned flying fox, Acerodon jubatus, which can weigh 1.6 kg and have a wingspan of 1.7 m.

Ecological light pollution

Ecological light pollution is the effect of artificial light on individual organisms and on the structure of ecosystems as a whole.

Eastern cattle egret species of bird

The eastern cattle egret is a species of heron found in the tropics, subtropics and warm temperate zones. Most taxonomic authorities lump this species and the western cattle egret together as subspecies of the cattle egret, but some separate them. Despite the similarities in plumage to the egrets of the genus Egretta, it is more closely related to the herons of Ardea. It is native to southern and eastern Asia, and Australasia.

Pollutant-induced abnormal behaviour refers to the abnormal behaviour induced by pollutants. Chemicals released into the natural environment by humans impact the behaviour of a wide variety of animals. The main culprits are endocrine-disrupting chemicals (EDCs), which mimic, block, or interfere with animal hormones. A new research field, integrative behavioural ecotoxicology, is emerging. However, chemical pollutants are not the only anthropogenic offenders. Noise and light pollution also induce abnormal behaviour.


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