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 a sleeping warthog a sleeping hyena main article: unihemispheric slow-wave sleep 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 sleeping grizzly bear 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. dreams dreaming in dogs has been studied by stanley coren, professor emeritus of psychology at the university of british columbia in vancouver. researchers have studied dreaming in dogs by manipulating the pons in the brain stem. he is the author of the book do dogs dream? nearly everything your dog wants you to know. (norton, 2012).