A sample hypnogram (electroencephalogram of sleep) showing sleep cycles characterized by increasing paradoxical <unk> . EEG of a mouse that shows<unk>  being characterized by prominent theta-rhythm<unk> <unk>  or<unk>  is a unique phase of sleep in mammals and birds, characterized by random<unk><unk>  of the<unk> s, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. The<unk>  phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep, because of physiological similarities to waking states including<unk> , low-voltage desynchronized brain waves. Electrical and chemical activity regulating this phase seems to originate in the brain stem, and is characterized most notably by an abundance of the neurotransmitter acetylcholine, combined with a nearly complete absence of monoamine neurotransmitters histamine, serotonin and norepinephrine.<unk>  is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep <unk> , NREMS, synchronized sleep).<unk>  and non-REM sleep alternate within one sleep cycle, which lasts about 90 minutes in adult humans. As sleep cycles continue, they shift towards a higher proportion of<unk> . The transition to<unk>  brings marked physical changes, beginning with electrical bursts called "ponto-geniculo-occipital waves" (PGO waves) originating in the brain stem. Organisms in<unk>  suspend central homeostasis, allowing large fluctuations in respiration, thermoregulation and circulation which do not occur in any other modes of sleeping or waking. The body abruptly loses muscle tone, a state known as<unk>  atonia. In 1953, Professor Nathaniel Kleitman and his student Eugene Aserinsky defined<unk> <unk><unk>  and linked it to dreams.<unk>  was further described by researchers, including William Dement and Michel Jouvet. Many experiments have involved awakening test subjects whenever they begin to enter the<unk>  phase, thereby producing a state known as<unk>  deprivation. Subjects allowed to sleep normally again usually experience a modest<unk>  rebound. Techniques of neurosurgery, chemical injection, electroencephalography, positron emission tomography, and reports of dreamers upon waking, have all been used to study this phase of sleep.
Rapid eye movement sleep
Twilight sleep
Sleep onset
Free-running sleep
Non-rapid eye movement sleep
Slow-wave sleep
Postictal state
Nightmare