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Rapid eye movement
In mammals and birds, sleep can be divided into two categories. In one of them, the eyes move rapidly. This is called REM-sleep (from rapid eye movement). Most dreams take place in this phase. REM-sleep occurs normally at intervals throughout the night. The periods of REM-sleep increase in length in the second half of the night. REM-sleep was first discovered in 1952-53. The function of REM sleep is not well understood.
Professor Nathaniel Kleitman and his student Eugene Aserinsky defined rapid eye movement and linked it to dreams in 1953. REM sleep was further described by researchers including William Dement and Michel Jouvet.
The other category, where this movement of the eyes does not happen, is called NREM-sleep (Non-REM sleep). Usually, dreams do not occur during this time. There are three or four stages of NREM-sleep. Stage I is just barely sleeping, or dozing. Stage II is also light sleep. Normally, in adult humans, about half of the time spent asleep is spent in light sleep. Stages III and IV are called deep sleep. Deep sleep is necessary for growth and healing. It can be quite difficult to awaken someone who is in stage III or stage IV sleep. Sometimes stages III and IV are combined and called stage III.
Adult humans normally sleep in cycles of 90 to 110 minutes each. The night's sleep can be four or fine of these cycles. Each cycle includes, in this order: stage I, stage II, stage III (IV), stage II and REM.
The fact that birds and mammals both show REM and NREM sleep suggests that the trait evolved before the two groups diverged in their evolution. That suggests that the REM/NREM feature is both early and important in the evolution of land vertebrates.
REM and dreams
Rapid eye movement sleep is associated with dreaming. Waking up sleepers during a REM phase is a common way to get dream reports. 80% of typical 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. They estimate the duration of their dreams as longer.
Lucid dreams are reported far more often in REM sleep. The mental events which occur during REM usually have dream 'hallmarks' including narrative structure, convincingness (like waking life), and instinctual themes.
Rapid eye movement sleep behavior disorder (RBD) is a sleep disorder (more specifically a parasomnia) in which people act out their dreams.
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