sleep in animals 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. sleep has been 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. a male and female lion sleeping on a flat plain definition a sleeping arctic fox part of a series onanimal dormancy torpor hibernation hibernaculum hit aestivation cryptobiosis brumation diapause embryonic diapause winter rest critical thermal maximum sleep developmental biology 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, 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 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 hence sleep is essential for all living species. sleep helps the body and mind to feel rested. findings show that if rats do not get sleep they die a in few weeks. despite having enough food, their appetite tends to decrease resulting in weight loss and eventually death. 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 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 fresh-water polyp hydra vulgaris and the jellyfish cassiopea are among the most primitive organisms in which sleep-like states have been observed. observing sleep states in jellyfish provides evidence that sleep states do not require that an animal have a brain or central nervous system. 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. a cuckoo bee from the genus nomada, sleeping. note the characteristic position anchored by the mandibles. american native bees, tribe eucerini in this case, frequently bed down in flower blossoms. bees have some of the most complex sleep states amongst insects. 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 gene expression studies of hendricks et al. 2000 and shaw et al. 2000 showing orthology between mammals and the fruit fly drosophila melanogaster for this to finally be accepted. 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 shown that they use sleep to store long-term memories. sleep-like state has been described in jumping spiders, too, as well as regularly occurring bouts of retinal movements that suggest an rem sleep–like state. also sleeping cuttlefish and octopuses show signs of having rem-sleep behaviors. fish sleep in fish is the subject of ongoing 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. studies show that some fish (for example rays and sharks) have unihemispheric sleep which means they put their half brain to sleep and the other half still remains active and they swim while they are sleeping. 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. sleeping african dwarf fischer's chameleon 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. 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. a komodo dragon sleeping in 2016, a study 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. a flamingo with at least one cerebral hemisphere awake 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 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, 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 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 phenomenon of unihemispheric slow-wave sleep; that is, the ability to sleep with one cerebral hemisphere at a time, while keeping other hemisphere awake. when just 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 by the level of 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 cats sleeping together sleeping japanese macaques sleeping binturong sleeping and resting horses 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