<P> After the deprivation is complete, mild psychological disturbances, such as anxiety, irritability, hallucinations, and difficulty concentrating may develop and appetite may increase . There are also positive consequences of REM deprivation . Some symptoms of depression are found to be suppressed by REM deprivation; aggression, and eating behavior may increase . Higher norepinepherine is a possible cause of these results . Whether and how long - term REM deprivation has psychological effects remains a matter of controversy . Several reports have indicated that REM deprivation increases aggression and sexual behavior in laboratory test animals . Rats deprived of paradoxical sleep die in 4--6 weeks (twice the time before death in case of total sleep deprivation). Mean body temperature falls continually during this period . </P> <P> It has been suggested that acute REM sleep deprivation can improve certain types of depression when depression appears to be related to an imbalance of certain neurotransmitters . Although sleep deprivation in general annoys most of the population, it has repeatedly been shown to alleviate depression, albeit temporarily . More than half the individuals who experience this relief report it to be rendered ineffective after sleeping the following night . Thus, researchers have devised methods such as altering the sleep schedule for a span of days following a REM deprivation period and combining sleep - schedule alterations with pharmacotherapy to prolong this effect . Antidepressants (including selective serotonin reuptake inhibitors, tricyclics, and monoamine oxidase inhibitors) and stimulants (such as amphetamine, Cannabis and cocaine) interfere with REM sleep by stimulating the monoamine neurotransmitters which must be suppressed for REM sleep to occur . Administered at therapeutic doses, these drugs may stop REM sleep entirely for weeks or months . Withdrawal causes a REM rebound . Sleep deprivation stimulates hippocampal neurogenesis much as antidepressants do, but whether this effect is driven by REM sleep in particular is unknown . </P> <P> Although it manifests differently in different animals, REM sleep or something like it occurs in all land mammals as well as in birds . The primary criteria used to identify REM are the change in electrical activity, measured by EEG, and loss of muscle tone, interspersed with bouts of twitching in phasic REM . The amount of REM sleep and cycling varies among animals; predators enjoy more REM sleep than prey . Larger animals also tend to stay in REM for longer, possibly because higher thermal inertia of their brains and bodies allows them to tolerate longer suspension of thermoregulation . The period (full cycle of REM and non-REM) lasts for about 90 minutes in humans, 22 minutes in cats, and 12 minutes in rats . In utero, mammals spend more than half (50--80%) of a 24 - hour day in REM sleep . </P> <P> Sleeping reptiles do not seem to have PGO waves or the localized brain activation seen in mammalian REM . However they do exhibit sleep cycles with phases of REM - like electrical activity measurable by EEG . A recent study found periodic eye movements in the central bearded dragon of Australia, leading its authors to speculate that the common ancestor of amniotes may therefore have manifested some precursor to REMS . </P>

Which group experiences the highest level of rem sleep
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