Sleep in animals
Sleep in animals refers to a behavioral and physiological state characterized by altered consciousness, reduced responsiveness to external stimuli, and homeostatic regulation observed in various animals.
Definition
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Animal dormancy |
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Sleep can follow a
Necessity
If sleep were not essential, one would expect to find
- Animal species that do not sleep at all
- Animals that do not need recovery sleep after staying awake longer than usual
- Animals that suffer no serious consequences as a result of lack of sleep
Hence sleep is essential for all complex animals. Sleep helps the body and mind to feel rested. Findings show that if rats do not get sleep, they die in a few weeks. Despite having enough food, their appetite tends to decrease resulting in weight loss and eventually death.[5]
Outside of a few basal animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria.[6] While some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. However, it remains to be shown definitively whether any fish is capable of unihemispheric sleep.[7]
Invertebrates
Sleep as a phenomenon appears to have very old evolutionary roots. Unicellular organisms do not necessarily "sleep", although many of them have pronounced
Bees have some of the most complex sleep states amongst insects.[12] Decade after decade results mounted that insects do sleep, and that this resembles mammalian and avian sleep. Nonetheless, sleep scientists continued to not accept these results and there was wide agreement that insects did not experience sleep. It took the
Fish
Sleep in fish is the subject of ongoing scientific research.[22][23] Typically fish exhibit periods of inactivity but show no significant reactions to deprivation of this condition.[inconsistent] 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.[24] There is also doubt about certain blind species that live in caves.[25] Other fish seem to sleep, however. For example, zebrafish,[26][27] tilapia,[28] tench,[29] brown bullhead,[30] and swell shark[31] become motionless and unresponsive at night (or by day, in the case of the swell shark); Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response.[32] Studies show that some fish (for example rays and sharks) have unihemispheric sleep, which means they put half their brain to sleep while the other half still remains active and they swim while they are sleeping.[7][33] A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep.[34] On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care.[35]
Land vertebrates
Mammals, birds and reptiles evolved from amniotic ancestors, the first vertebrates with life cycles independent of water. The fact that birds and mammals are the only known animals to exhibit REM and NREM sleep indicates a common trait before divergence.[36] However, recent evidence of REM-like sleep in fish suggests this divergence may have occurred much earlier than previously thought.[37] Up to this point, reptiles were considered the most logical group to investigate the origins of sleep. Daytime activity in reptiles alternates between basking and short bouts of active behavior, 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.[38]
Reptiles and amphibians
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.[4] 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.[39]
In 2016, a study[40] reported the existence of REM- and NREM-like sleep stages in the Australian dragon Pogona vitticeps. Amphibians have periods of inactivity but show high vigilance (receptivity to potentially threatening stimuli) in this state.
Like some birds and aquatic mammals, crocodilians are also capable of unihemispheric sleep.[41]
Birds
There are significant similarities between sleep in
Birds have both REM and NREM sleep, and the EEG patterns of both have similarities to those of mammals. Different birds sleep different amounts, but the associations seen in mammals between sleep and variables such as body mass, brain mass, relative brain mass, basal metabolism and other factors (see below) are not found in birds. 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.[45]
Birds do not necessarily exhibit sleep debt, but a peculiarity that birds share with aquatic mammals, and possibly also with certain species of
Opinions partly differ about sleep in
Mammals
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
Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%.
Nocturnal animals have higher body temperatures, greater activity, rising serotonin, and diminishing cortisol during the night—the inverse of diurnal animals. Nocturnal and diurnal animals both have increased electrical activity in the suprachiasmatic nucleus, and corresponding secretion of melatonin from the pineal gland, at night.[55] Nocturnal mammals, which tend to stay awake at night, have higher melatonin at night just like diurnal mammals do.[56] And, although removing the pineal gland in many animals abolishes melatonin rhythms, it does not stop circadian rhythms altogether—though it may alter them and weaken their responsiveness to light cues.[57] Cortisol levels in diurnal animals typically rise throughout the night, peak in the awakening hours, and diminish during the day.[58][59] In diurnal animals, sleepiness increases during the night.
Duration
Different mammals sleep different amounts. Some, such as bats, sleep 18–20 hours per day, while others, including giraffes, sleep only 3–4 hours per day. There can be big differences even between closely related species. There can also be big differences between laboratory and field studies: for example, researchers in 1983 reported that captive sloths slept nearly 16 hours a day, but in 2008, when miniature neurophysiological recorders were developed that could be affixed to wild animals, sloths in nature were found to sleep only 9.6 hours a day.[60][61]
As with birds, the main rule for mammals (with certain exceptions, see below) is that they have two essentially different stages of sleep: REM and NREM sleep (see above). Mammals' feeding habits are associated with their sleep length. The daily need for sleep is highest in carnivores, lower in omnivores and lowest in herbivores. Humans sleep less than many other omnivores but otherwise not unusually much or unusually little in comparison with other mammals.[62]
Many herbivores, like
It has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.
- Comparative average sleep periods for various mammals (in captivity) over 24 hours[70]
- Horses – 2 hours[71]
- Elephants – 3+ hours [63]
- Cows– 4.0 hours
- Giraffes – 4.5 hours
- Humans – 8.0 hours
- Rabbits – 8.4 hours
- Chimpanzees– 9.7 hours
- Red foxes – 9.8 hours
- Dogs – 10.1 hours
- House mice – 12.5 hours
- Cats– 12.5 hours
- Lions – 13.5 hours
- Platypuses – 14 hours
- Chipmunks – 15 hours
- Tigers– 15.8 hours
- Giant armadillos – 18.1 hours
- Leopards – 18 hours
- Little brown bats – 19.9 hours
Reasons given for the wide variations include the fact that mammals "that nap in hiding, like bats or rodents tend to have longer, deeper snoozes than those on constant alert." Lions, which have little fear of predators also have relatively long sleep periods, while elephants have to eat most of the time to support their huge bodies. Little brown bats conserve their energy except for the few hours each night when their insect prey are available, and platypuses eat a high energy crustacean diet and, therefore, probably do not need to spend as much time awake as many other mammals.[72]
Rodents
A study conducted by Datta indirectly supports the idea that memory benefits from sleep.[73] A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. During the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials.[73]
An observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session.
During each of these three periods,
In rats, sleep deprivation causes weight loss and reduced body temperature. Rats kept awake indefinitely develop skin lesions,
A 2014 study found that depriving mice of sleep increased cancer growth and dampened the immune system's ability to control cancers. The researchers found higher levels of M2
Monotremes
Since
Aquatic mammals
The consequences of falling into a deep sleep for marine mammalian species can be suffocation and drowning, or becoming easy prey for predators. Thus, dolphins, whales, and pinnipeds (seals) engage in unihemispheric sleep while swimming, which allows one brain hemisphere to remain fully functional, while the other goes to sleep. The hemisphere that is asleep alternates, so that both hemispheres can be fully rested.[68][82] Just like terrestrial mammals, pinnipeds that sleep on land fall into a deep sleep and both hemispheres of their brain shut down and are in full sleep mode.[83][84] Aquatic mammal infants do not have REM sleep in infancy;[85] REM sleep increases as they age.
Among others, seals and whales belong to the aquatic mammals. Earless seals and eared seals have solved the problem of sleeping in water via two different methods. Eared 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.
Earless 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.[86] 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.[62][87] 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.[88]
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.[89]
Unihemispherism
Unihemispheric sleep refers to sleeping with only a single
The
The fact that unihemispheric sleep exists has been used as an argument for the necessity of sleep.[92] It appears that no animal has developed an ability to go without sleep altogether.
Hibernation
Animals that
Dreams
Dreaming in dogs has been studied by
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