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How do animals sleep? This is a question some children asked me recently. 

This answer can be written as a book as the animal kingdom is huge. Anyway, I will give only the important  information. 

The definition of sleep can follow a physiological or behavioral pattern (1). In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone, and a compensatory increase following deprivation of the state, this last known as sleep homeostasis (i.e., the longer a waking state lasts, the greater the intensity and duration of the sleep state thereafter). In the behavioral sense, sleep is characterized by minimal movement, non-responsiveness to external stimuli (i.e. increased sensory threshold), the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. The physiological definition applies well to birds and mammals, but in other animals (whose brain is not as complex), the behavioral definition is more often used.

In very simple animals, behavioral definitions of sleep are the only ones possible, and even then the behavioural repertoire of the animal may not be extensive enough to allow distinction between sleep and wakefulness. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep.  It appears that no (especially among the highly developed ones) animal has developed an ability to go without sleep altogether.

Some insects are active at night to take advantage of stealth dining. Cutworms eat leaves when the sun goes down to avoid birds and other predators, and bed bugs usually feed at night to take advantage their meal as it sleeps . By comparison, insects that forage for their food in the daytime, such as various bee species seeking out pollen, are on the opposite circadian cycle. Which makes sense if the flowers they pollinate close up at night.

Insects go through circadian rhythms of activity and passivity but some   do not seem to have a homeostatic sleep need. Insects do not seem to exhibit REM sleep. However, fruit flies appear to,sleep,  and systematic disturbance of that state leads to cognitive disabilities. Still, measuring sleep in insects is tricky—it's not always easy, for instance, to differentiate between sleep and sleep-like states.

Bugs don’t sleep in quite the same way that we humans do. For example, insects don’t have eyelids, so they can’t get any “shut-eye”. Scientists usually study sleep in other animals by observing brain activity. They haven’t been able to do so with insects, though. Still, they think that insects do indeed rest each day. They just do it a little differently than we do.

Most insects are either active only during the day or only at night. When they’re not active, they rest. This state of rest in insects is called torpor, and it’s not exactly like sleep as we know it.

During torpor, insects remain very still and don’t respond much to stimuli around them. Insects in a state of torpor can appear to be sleeping because they aren’t moving or responding to the world around them.

Insects can come out of torpor in a matter of seconds if an environmental stimulus is powerful enough. Stimuli that can “wake” insects out of torpor include sounds, movement, and the rising (or setting) of the Sun (6).

Flies exhibit key features of sleep. For example, flies at rest 're harder to startle than those that where active, just as it’s harder to get your attention when you’re snoozing on the couch than when you’re up and moving around. 

So, where do insects rest when they’re in a state of torpor? It can be just about anywhere they feel comfortable and safe from predators. Some resting places can be a bit strange. For example, some bees will clamp their jaws onto a plant and fold up their legs as they enter a state of torpor, dangling in this odd pose until morning!

Bees do sleep: Watch here how they do it....

Signs of true bug  sleep are not moving, "drooping in the direction of gravity," and more relaxed muscles. Another indicator is "increased arousal threshold," or how long it takes to jar the bug to alertness.

Paper wasps, cockroaches, paying mantises, and fruit flies are among insects that doze. Fruit fly sleep is even similar to mammal sleep.

Butterflies rest, but we don't know if they sleep, by  hanging from such hiding places as leaves, bark, or even beer cans.


Typically fish exhibit periods of inactivity. Some species that always live in shoals or that swim continuously (because of a need for ram ventilation of the gills, for example) are suspected never to sleep. There is also doubt about certain blind species that live in caves. Other fish seem to sleep, however. For example, zebrafish become motionless and unresponsive at night (or by day, in the case of the swell shark) in the water; Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response. A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep. On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care (2).

 
Daytime activity in reptiles alternates between basking and short bouts of active behaviour, which has significant neurological and physiological similarities to sleep states in mammals. It is proposed that REM sleep evolved from short bouts of motor activity in reptiles while Slow-Wave Sleep (SWS) evolved from their basking state which shows similar slow wave EEG patterns.  Reptiles have quiescent periods similar to mammalian sleep, and a decrease in electrical activity in the brain has been registered when the animals have been asleep. However, the EEG pattern in reptilian sleep differs from what is seen in mammals and other animals. In reptiles, sleep time increases following sleep deprivation, and stronger stimuli are needed to awaken the animals when they have been deprived of sleep as compared to when they have slept normally. This suggests that the sleep which follows deprivation is compensatorily deeper(3).
Amphibians have periods of inactivity but show high vigilance (receptivity to potentially threatening stimuli) in this state.

There are significant similarities between sleep in birds and sleep in mammals, which is one of the reasons for the idea that sleep in higher animals with its division into REM and NREM sleep has evolved together with warm-bloodedness.

Birds compensate for sleep loss in a manner similar to mammals, by deeper or more intense slow-wave sleep (SWS). Birds have both REM and NREM sleep, and the EEG patterns of both have similarities to those of mammals. Different birds sleep different amounts. The only clear explanatory factor for the variations in sleep amounts for birds of different species is that birds who sleep in environments where they are exposed to predators have less deep sleep than birds sleeping in more protected environments. Most Birds perch on branches and sleep. But Birds do not necessarily exhibit sleep debt, but a peculiarity that birds share with aquatic mammals, and possibly also with certain species of lizards (opinions differ about that last point), is the ability for unihemispheric sleep. That is the ability to sleep with one cerebral hemisphere at a time, while the other hemisphere is awake (Unihemispheric slow-wave sleep) (4). When only one hemisphere is sleeping, only the contralateral eye will be shut; that is, when the right hemisphere is asleep the left eye will be shut, and vice versa. The distribution of sleep between the two hemispheres and the amount of unihemispheric sleep are determined both by which part of the brain has been the most active during the previous period of wake—that part will sleep the deepest—and it is also determined by the risk of attacks from predators. Ducks near the perimeter of the flock are likely to be the ones that first will detect predator attacks. These ducks have significantly more unihemispheric sleep than those who sleep in the middle of the flock, and they react to threatening stimuli seen by the open eye..

Opinions partly differ about sleep in migratory birds.The controversy is mainly about whether they can sleep while flying or not. Theoretically, certain types of sleep could be possible while flying, but technical difficulties preclude the recording of brain activity in birds while they are flying.

Mammals have wide diversity in sleep phenomena. Generally, they go through periods of alternating non-REM and REM sleep, but these manifest differently. Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Male armadillos do get erections during non-REM sleep, and the inverse is true in rats.

Early mammals engaged in polyphasic sleep, dividing sleep into multiple bouts per day. Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%.Additionally it is observed that mammals require sleep even during the hypometabolic state of hibernation, in which circumstance it is actually a net loss of energy as the animal returns from hypothermia to euthermia in order to sleep.Many herbivores, such as cattle, spend much of their wake time in a state of drowsiness.  Mammals usually lie down. Mammals born with well-developed regulatory systems, such as the horse and giraffe, tend to have less REM sleep than the species which are less developed at birth, such as cats and rats. This appears to echo the greater need for REM sleep among newborns than among adults in most mammal species. Many mammals sleep for a large proportion of each 24-hour period when they are very young. The giraffe only sleeps 2 hours a day in about 5–15 minute sessions. Koalas are the longest sleeping-mammals, about 20–22 hours a day. However, killer whales and some other dolphins do not sleep during the first month of life. Instead, young dolphins and whales frequently take rests by pressing their body next to their mother's while she swims. As the mother swims she is keeping her offspring afloat to prevent them from drowning.  During this period, mothers often sacrifice sleep for the protection of their young from predators. However, unlike other mammals, adult dolphins and whales are able to go without sleep for a month.
Some seals, like whales, show unihemispheric sleep. The sleeping half of the brain does not awaken when they surface to breathe. When one half of a seal's brain shows slow-wave sleep, the flippers and whiskers on its opposite side are immobile. While in the water, these seals have almost no REM sleep and may go a week or two without it. As soon as they move onto land they switch to bilateral REM sleep and NREM sleep comparable to land mammals, surprising researchers with their lack of "recovery sleep" after missing so much REM. Cape fur seal, asleep in a zooEarless seals sleep bihemispherically like most mammals, under water, hanging at the water surface or on land. They hold their breath while sleeping under water, and wake up regularly to surface and breathe. They can also hang with their nostrils above water and in that position have REM sleep, but they do not have REM sleep underwater.

REM sleep has been observed in the pilot whale, a species of dolphin. Whales do not seem to have REM sleep, nor do they seem to have any problems because of this. One reason REM sleep might be difficult in marine settings is the fact that REM sleep causes muscular atony; that is to say, a functional paralysis of skeletal muscles that can be difficult to combine with the need to breathe regularly.Conscious breathing cetaceans sleep but cannot afford to be unconscious for long, because they may drown. While knowledge of sleep in wild cetaceans is limited, toothed cetaceans in captivity have been recorded to exhibit unihemispheric slow-wave sleep (USWS), which means they sleep with one side of their brain at a time, so that they may swim, breathe consciously and avoid both predators and social contact during their period of rest

A 2008 study found that sperm whales sleep in vertical postures just under the surface in passive shallow 'drift-dives', generally during the day, during which whales do not respond to passing vessels unless they are in contact, leading to the suggestion that whales possibly sleep during such dives.

Unihemispheric sleep refers to sleeping with only a single cerebral hemisphere. The phenomenon has been observed in birds and aquatic mammals,as well as in several reptilian species (the latter being disputed: many reptiles behave in a way which could be construed as unihemispheric sleeping, but EEG studies have given contradictory results). Reasons for the development of unihemispheric sleep are likely that it enables the sleeping animal to receive stimuli—threats, for instance—from its environment, and that it enables the animal to fly or periodically surface to breathe when immersed in water. Only NREM sleep exists unihemispherically, and there seems to exist a continuum in unihemispheric sleep regarding the differences in the hemispheres: in animals exhibiting unihemispheric sleep, conditions range from one hemisphere being in deep sleep with the other hemisphere being awake to one hemisphere sleeping lightly with the other hemisphere being awake. If one hemisphere is selectively deprived of sleep in an animal exhibiting unihemispheric sleep (one hemisphere is allowed to sleep freely but the other is awoken whenever it falls asleep), the amount of deep sleep will selectively increase in the hemisphere that was deprived of sleep when both hemispheres are allowed to sleep freely.The neurobiological background for unihemispheric sleep is still unclear. In experiments on cats in which the connection between the left and the right halves of the brain stem has been severed, the brain hemispheres show periods of a desynchronized EEG, during which the two hemispheres can sleep independently of each other(5). In these cats, the state where one hemisphere slept NREM and the other was awake, as well as one hemisphere sleeping NREM with the other state sleeping REM were observed. The cats were never seen to sleep REM sleep with one hemisphere while the other hemisphere was awake. This is in accordance with the fact that REM sleep, as far as is currently known, does not occur unihemispherically. The fact that unihemispheric sleep exists has been used as an argument for the necessity of sleep. 

Footnotes:

1. https://en.wikipedia.org/wiki/Sleep_in_non-human_animals#:~:text=In....

2. Reebs, S.G. (2002). "Plasticity of diel and circadian activity rhythms in fish". Reviews in Fish Biology and Fisheries12 (4): 349–371. doi:10.1023/a:1025371804611S2CID 33118836.

3. Flanigan WF (1973). "Sleep and wakefulness in iguanid lizards, Ctenosaura pectinata and Iguana iguana". Brain Behav. Evol8 (6): 401–36. doi:10.1159/000124366PMID 4802023.

4. Rattenborg, NC; Amlaner, CJ; Lima, SL (2000). "Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep;". Neurosci Biobehav Rev24 (8): 817–42. doi:10.1016/s0149-7634(00)00039-7PMID 11118608S2CID 7592942.

5. Michel, F.; Roffwarg, H.P. (February 1967). "Chronic split brainstem preparation: effect on sleep–waking cycle". Experientia (in French). Basel: Birkhäuser. 23 (2): 126–128. doi:10.1007/BF02135958PMID 6032104S2CID 37925278.

6. https://wonderopolis.org/wonder/where-do-bugs-sleep

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