rem phase is a common experimental method for obtaining dream reports; 80% of neurotypical people can give some kind of dream report under these circumstances. sleepers awakened from rem tend to give longer, more narrative descriptions of the dreams they were experiencing, and to estimate the duration of their dreams as longer. lucid dreams are reported far more often in rem sleep. (in fact these could be considered a hybrid state combining essential elements of rem sleep and waking consciousness.) the mental events which occur during rem most commonly have dream hallmarks including narrative structure, convincingness (e.g., experiential resemblance to waking life), and incorporation of instinctual themes. sometimes, they include elements of the dreamer's recent experience taken directly from episodic memory. by one estimate, 80% of dreams occur during rem. hobson and mccarley proposed that the pgo waves characteristic of "phasic" rem might supply the visual cortex and forebrain with electrical excitement which amplifies the hallucinatory aspects of dreaming. however, people woken up during sleep do not report significantly more bizarre dreams during phasic rems, compared to tonic rems. another possible relationship between the two phenomena could be that the higher threshold for sensory interruption during rem sleep allows the brain to travel further along unrealistic and peculiar trains of thought. some dreaming can take place during non-rem sleep. "light sleepers" can experience dreaming during stage 2 non-rem sleep, whereas "deep sleepers", upon awakening in the same stage, are more likely to report "thinking" but not "dreaming". certain scientific efforts to assess the uniquely bizarre nature of dreams experienced while asleep were forced to conclude that waking thought could be just as bizarre, especially in conditions of sensory deprivation. because of non-rem dreaming, some sleep researchers have strenuously contested the importance of connecting dreaming to the rem sleep phase. the prospect that well-known neurological aspects of rem do not themselves cause dreaming suggests the need to re-examine the neurobiology of dreaming per se. some researchers (dement, hobson, jouvet, for example) tend to resist the idea of disconnecting dreaming from rem sleep. effects of ssris previous research has shown that selective serotonin reuptake inhibitors (ssris) have an important effect on rem sleep neurobiology and dreaming. a study at harvard medical school in 2000 tested the effects of paroxetine and fluvoxamine on healthy young adult male and females for 31 days: a drug-free baseline week, 19 days on either paroxetine or fluvoxamine with morning and evening doses, and 5 days of absolute discontinuation. results showed that ssri treatment decreased the average amount of dream recall frequency in comparison to baseline measurements as a result of serotonergic rem suppression. fluvoxamine increased the length of dream reporting, bizarreness of dreams as well as the intensity of rem sleep. these effects were the greatest during acute discontinuation compared to treatment and baseline days. however, the subjective intensity of dreaming increased and the proclivity to enter rem sleep was decreased during ssri treatment compared to baseline and discontinuation days. creativity after waking from rem sleep, the mind seems "hyperassociative"—more receptive to semantic priming effects. people awakened from rem have performed better on tasks like anagrams and creative problem solving. sleep aids the process by which creativity forms associative elements into new combinations that are useful or meet some requirement. this occurs in rem sleep rather than in nrem sleep. rather than being due to memory processes, this has been attributed to changes during rem sleep in cholinergic and noradrenergic neuromodulation. high levels of acetylcholine in the hippocampus suppress feedback from hippocampus to the neocortex, while lower levels of acetylcholine and norepinephrine in the neocortex encourage the uncontrolled spread of associational activity within neocortical areas. this is in contrast to waking consciousness, where higher levels of norepinephrine and acetylcholine inhibit recurrent connections in the neocortex. rem sleep through this process adds creativity by allowing "neocortical structures to reorganise associative hierarchies, in which information from the hippocampus would be reinterpreted in relation to previous semantic representations or nodes." timing sample hypnogram (electroencephalogram of sleep) showing sleep cycles characterized by increasing paradoxical (rem) sleep. in the ultradian sleep cycle, an organism alternates between deep sleep (slow, large, synchronized brain waves) and paradoxical sleep (faster, desynchronized waves). sleep happens in the context of the larger circadian rhythm, which influences sleepiness and physiological factors based on timekeepers within the body. sleep can be distributed throughout the day or clustered during one part of the rhythm: in nocturnal animals, during the day, and in diurnal animals, at night. the organism returns to homeostatic regulation almost immediately after rem sleep ends. during a night of sleep, humans usually experience about four or five periods of rem sleep; they are shorter (~15 min) at the beginning of the night and longer (~25 min) toward the end. many animals and some people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of rem. the relative amount of rem sleep varies considerably with age. a newborn baby spends more than 80% of total sleep time in rem. rem sleep typically occupies 20–25% of total sleep in adult humans: about 90–120 minutes of a night's sleep. the first rem episode occurs about 70 minutes after falling asleep. cycles of about 90 minutes each follow, with each cycle including a larger proportion of rem sleep. (the increased rem sleep later in the night is connected with the circadian rhythm and occurs even in people who did not sleep in the first part of the night.) in the weeks after a human baby is born, as its nervous system matures, neural patterns in sleep begin to show a rhythm of rem and non-rem sleep. (in faster-developing mammals, this process occurs in utero.) infants spend more time in rem sleep than adults. the proportion of rem sleep then decreases significantly in childhood. older people tend to sleep less overall, but sleep in rem for about the same absolute time (and therefore spend a greater proportion of sleep in rem). rapid eye movement sleep can be subclassified into tonic and phasic modes. tonic rem is characterized by theta rhythms in the brain; phasic rem is characterized by pgo waves and actual "rapid" eye movements. processing of external stimuli is heavily inhibited during phasic rem, and recent evidence suggests that sleepers are more difficult to arouse from phasic rem than in slow-wave sleep. deprivation effects selective rems deprivation causes a significant increase in the number of attempts to go into rem stage while asleep. on recovery nights, an individual will usually move to stage 3 and rem sleep more quickly and experience a rem rebound, which refers to an increase in the time spent in rem stage over normal levels. these findings are consistent with the idea that rem sleep is biologically necessary. however, the "rebound" rem sleep usually does not last fully as long as the estimated length of the missed rem periods. 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 may increase, and eating behavior may get disrupted. 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. 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, methylphenidate 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. in other animals main article: sleep in non-human animals 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 experience 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. 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. observations of jumping spiders in their nocturnal resting position also suggest a rem sleep-like state characterized by bouts of twitching and retinal movements and hints of muscle atonia (legs curling up as a result of pressure loss caused by muscle atonia in the prosoma). sleep deprivation experiments on non-human animals can be set up differently than those on humans. the "flower pot" method involves placing a laboratory animal above water on a platform so small that it falls off upon losing muscle tone. the naturally rude awakening which results may elicit changes in the organism which necessarily exceed the simple absence of a sleep phase. this method also stops working after about 3 days as the subjects (typically rats) lose their will to avoid the water. another method involves computer monitoring of brain waves, complete with automatic mechanized shaking of the cage when the test animal drifts into rem sleep. possible functions some researchers argue that the perpetuation of a complex brain process such as rem sleep indicates that it serves an important function for the survival of mammalian and avian species. it fulfills important physiological needs vital for survival to the extent that prolonged rem sleep deprivation leads to death in experimental animals. in both humans and experimental animals, rem sleep loss leads to several behavioral and physiological abnormalities. loss of rem sleep has been noticed during various natural and experimental infections. survivability of the experimental animals decreases when rem sleep is totally attenuated during infection; this leads to the possibility that the quality and quantity of rem sleep is generally essential for normal body physiology. further, the existence of a "rem rebound" effect suggests the possibility of a biological need for rem sleep. while the precise function of rem sleep is not well understood, several theories have been proposed. memory sleep in general aids memory. rem sleep may favor the preservation of certain types of memories: specifically, procedural memory, spatial memory, and emotional memory. in rats, rem sleep increases following intensive learning, especially several hours after, and sometimes for multiple nights. experimental rem sleep deprivation has sometimes inhibited memory consolidation, especially regarding complex processes (e.g., how to escape from an elaborate maze). in humans, the best evidence for rem's improvement of memory pertains to learning of procedures—new ways of moving the body (such as trampoline jumping), and new techniques of