A sleeping cat
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 (such as humans) and in
the day for nocturnal organisms (such as rodents). Sleep patterns vary
widely among species. It appears to be a requirement for all mammals and
most other animals.
Definition
Sleep can follow a physiological or behavioral definition. In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone
(possibly with some exceptions; see below regarding the sleep of birds
and of aquatic mammals), and a compensatory increase following
deprivation of the state. 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 behavioral
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.
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
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.
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.
Invertebrates
Caenorhabditis elegans is among the most primitive organisms in which sleep-like states have been observed.
A cuckoo bee from the genus Nomada, sleeping. Note the characteristic position anchored by the mandibles. Bees have some of the most complex sleep states amongst insects.
Sleep as a phenomenon appears to have very old evolutionary roots.
Unicellular organisms do not necessarily "sleep", although many of them
have pronounced circadian rhythms. The jellyfish Cassiopea is among the most primitive organism in which sleep-like states have been observed. The nematode C. elegans is another primitive organism that appears to require sleep. Here, a lethargus phase occurs in short periods preceding each moult, a fact which may indicate that sleep primitively is connected to developmental processes. Raizen et al.'s results furthermore suggest that sleep is necessary for changes in the neural system.
The electrophysiological study of sleep in small invertebrates
is complicated. 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.
There are several methods of measuring cognitive functions in fruit
flies. A common method is to let the flies choose whether they want to
fly through a tunnel that leads to a light source, or through a dark
tunnel. Normally, flies are attracted to light. But if sugar
is placed in the end of the dark tunnel, and something the flies
dislike is placed in the end of the light tunnel, the flies will
eventually learn to fly towards darkness rather than light. Flies
deprived of sleep require a longer time to learn this and also forget it
more quickly. If an arthropod is experimentally kept awake longer than it is used to, then its coming rest period will be prolonged. In cockroaches that rest period is characterized by the antennae being folded down and by a decreased sensitivity to external stimuli. Sleep has been described in crayfish, too, characterized by passivity and increased thresholds for sensory stimuli as well as changes in the EEG pattern, markedly differing from the patterns found in crayfish when they are awake. In honeybees, it has been suggested they could be able to dream.
Fish
Sleep in fish is subject of current scientific research.
Typically fish exhibit periods of inactivity but show no significant
reactions to deprivation of this condition. 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, tilapia, tench, brown bullhead, and swell shark 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. 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.
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.
However, recent evidence of REM-like sleep in fish suggests this
divergence may have occurred much earlier than previously thought.
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.
Reptiles and amphibians
Sleeping African dwarf Fischer's chameleon
A Komodo dragon sleeping.
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.
In 2016, a study report 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.
Birds
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 SWS (slow-wave sleep).
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.
A sleeping cockatiel
A flamingo with at least one cerebral hemisphere awake
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).
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
Sleeping Japanese macaques.
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 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. Higher daily sleep quotas and shorter sleep cycles in
polyphasic species as compared to monophasic species, suggest that
polyphasic sleep may be a less efficient means of attaining sleep’s
benefits. Small species with higher BMR may therefore have less
efficient sleep patterns. It follows that the evolution of monophasic
sleep may hitherto be an unknown advantage of evolving larger mammalian
body sizes and therefore lower BMR.
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.
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. Nocturnal mammals, which tend to stay awake at night, have higher melatonin at night just like diurnal mammals do.
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. Cortisol levels in diurnal animals typically rise throughout the night, peak in the awakening hours, and diminish during the day. In diurnal animals, sleepiness increases during the night.
Duration
Flying foxes, asleep
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 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.
Sleeping polar bears
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.
Many herbivores, like Ruminantia (such as cattle), spend much of their wake time in a state of drowsiness,
which perhaps could partly explain their relatively low need for sleep.
In herbivores, an inverse correlation is apparent between body mass and
sleep length; big mammals sleep less than smaller ones. This
correlation is thought to explain about 25% of the difference in sleep
amount between different mammals.
Also, the length of a particular sleep cycle is associated with the
size of the animal; on average, bigger animals will have sleep cycles of
longer durations than smaller animals. Sleep amount is also coupled to
factors like basal metabolism, brain mass, and relative brain mass. The duration of sleep among species is also directly related to basal metabolic rate
(BMR). Rats, which have a high BMR, sleep for up to 14 hours a day,
whereas elephants and giraffes, which have lower BMRs, sleep only 3–4
hours per day.
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. 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. This
allows young dolphins and whales to rest, which will help keep their
immune system healthy; in turn, protecting them from illnesses.
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.
- Comparative average sleep periods for various mammals (in captivity) over 24 hours.
A sleeping dog
- Horses – 2 hours
- Elephants – 3+ hours
- 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
- Giant armadillos – 18.1 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.
Rodents
A sleeping rat
A study conducted by Datta indirectly supports the idea that memory benefits from sleep.[57]
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.
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. This study shows that after spatial exploration activity, patterns of hippocampal
place cells are reactivated during SWS following the experiment. Rats
were run through a linear track using rewards on either end. The rats
would then be placed in the track for 30 minutes to allow them to adjust
(PRE), then they ran the track with reward-based training for 30
minutes (RUN), and then they were allowed to rest for 30 minutes.
During each of these three periods, EEG
data were collected for information on the rats' sleep stages. The mean
firing rates of hippocampal place cells during prebehavior SWS (PRE)
and three ten-minute intervals in postbehavior SWS (POST) were
calculated by averaging across 22 track-running sessions from seven
rats. The results showed that ten minutes after the trial RUN session,
there was a 12% increase in the mean firing rate of hippocampal place
cells from the PRE level. After 20 minutes, the mean firing rate
returned rapidly toward the PRE level. The elevated firing of
hippocampal place cells during SWS after spatial exploration could
explain why there were elevated levels of slow-wave sleep in Datta's
study, as it also dealt with a form of spatial exploration.
In rats, sleep deprivation causes weight loss and reduced body temperature. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and, eventually, fatal sepsis. Sleep deprivation also hinders the healing of burns on rats. When compared with a control group, sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system.
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 tumor-associated macrophages and TLR4
molecules in the sleep deprived mice and proposed this as the mechanism
for increased susceptibility of the mice to cancer growth. M2 cells
suppress the immune system and encourage tumour growth. TRL4 molecules
are signalling molecules in the activation of the immune system.
Monotremes
Since monotremes
(egg-laying mammals) are considered to represent one of the
evolutionarily oldest groups of mammals, they have been subject to
special interest in the study of mammalian sleep. As early studies of
these animals could not find clear evidence for REM sleep, it was
initially assumed that such sleep did not exist in monotremes, but
developed after the monotremes branched off from the rest of the
mammalian evolutionary line, and became a separate, distinct group.
However, EEG recordings of the brain stem in monotremes show a firing pattern that is quite similar to the patterns seen in REM sleep in higher mammals. In fact, the largest amount of REM sleep known in any animal is found in the platypus.
REM electrical activation does not extend at all to the forebrain in
platypods, suggesting that they do not dream. The average sleep time of
the platypus in a 24-hour period is said to be as long as 14 hours,
though this may be because of their high-calorie crustacean diet.
Aquatic mammals
Northern sea lion pup with adult female and male, the largest of the eared seals. Habitat: the northern Pacific.
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.
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. Aquatic mammal infants do not have REM sleep in infancy; 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.
Cape fur seal, asleep in a zoo
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.
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.
Unihemispherism
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.
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. It appears that no animal has developed an ability to go without sleep altogether.
Hibernation
Animals that hibernate are in a state of torpor,
differing from sleep. Hibernation markedly reduces the need for sleep,
but does not remove it. Some hibernating animals end their hibernation a
couple of times during the winter so that they can sleep. Hibernating
animals waking up from hibernation often go into rebound sleep because
of lack of sleep during the hibernation period. They are definitely
well-rested and are conserving energy during hibernation, but need sleep
for something else.
