less efficient means of attaining sleep's benefits. small species with higher basal metabolic rate (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. flying foxes, asleep 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. 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 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 2–4 hours per day. a sleeping snow leopard 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 red fox a sleeping dog a sleeping tiger a sleeping leopard 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 tigers – 15.8 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 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. rodents a sleeping rat a study conducted by datta indirectly supports the idea that memory benefits from sleep. 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 a platypus sleeping in the water before dark 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