<unk> <unk><unk>  <unk>  is the normal stage of sleep characterized by<unk><unk> s of the<unk> s.<unk>  is classified into two categories: tonic and phasic. It was discovered by Nathaniel Kleitman and Eugene Aserinsky in the early 1950s.  Their seminal article was published September 4, 1953.  Criteria for<unk>  include not only<unk> <unk><unk> s, but also low muscle tone and a<unk> , low voltage EEG -- these features are easily discernible in a polysomnogram, the sleep study typically done for patients with suspected sleep disorders.<unk>  in adult humans typically occupies 20-25% of total sleep, lasting about 90-120 minutes.  During a normal night of sleep, humans usually experience about 4 or 5 periods of<unk> ; they are quite short at the beginning of the night and longer toward the end. It is common for one to wake for a short time at the end of an<unk>  phase. The relative amount of<unk>  varies considerably with age. A newborn baby spends more than 80% of total sleep time in<unk>  (see also Active Sleep). During<unk> , the summed activity of the brain's neurons is a quite similar to that during waking hours; for this reason, the phenomenon is often called paradoxical sleep.  This means that there are no dominating brain waves during<unk> .<unk>  is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep. Most of our vividly recalled dreams occur during<unk> .
Rapid eye movement sleep
Postictal state
Nightmare
Sleep onset latency
Golden hour 
Refractory period
Face time
Disaster response