Sleep in animals
[1][2] 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 fresh-water polyp Hydra vulgaris and the jellyfish Cassiopea are among the most primitive organisms in which sleep-like states have been observed.
Here, a lethargus phase occurs in short periods preceding each moult, a fact which may indicate that sleep primitively is connected to developmental processes.
However, fruit flies appear to sleep, and systematic disturbance of that state leads to cognitive disabilities.
In cockroaches, that rest period is characterized by the antennae being folded down and by a decreased sensitivity to external stimuli.
[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.
[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] 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.
[38] 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.
[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.
It follows that the evolution of monophasic sleep may hitherto be an unknown advantage of evolving larger mammalian body sizes and therefore lower BMR.
[51] 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 and diurnal animals both have increased electrical activity in the suprachiasmatic nucleus, and corresponding secretion of melatonin from the pineal gland, at night.
[56] Cortisol levels in diurnal animals typically rise throughout the night, peak in the awakening hours, and diminish during the day.
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.
[63] 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 allows young dolphins and whales to rest, which will help keep their immune system healthy; in turn, protecting them from illnesses.
[67][68] 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.
[73] This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment.
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.
Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and, eventually, fatal sepsis.
[71] The consequences of falling into a deep sleep for marine mammalian species can be suffocation and drowning, or becoming easy prey for predators.
When one half of a seal's brain shows slow-wave sleep, the flippers and whiskers on its opposite side are immobile.
The phenomenon has been observed in birds and aquatic mammals,[89] 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).
