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Physiology
In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM sleep) and non-rapid eye movement (NREM or non-REM sleep). Each type has a distinct set of associated physiological and neurological features. REM sleep is associated with the capability of dreaming. The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep.

Stages

 * REM: The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete serotonin. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage. Vital signs indicate arousal and oxygen consumption by the brain is higher than when the sleeper is awake. An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. REM sleep occurs as a person goes from NREM stage 2 to NREM stage 1. The function of REM sleep is uncertain but a lack of it will impair the ability to learn complex tasks.

Dreaming
Dreaming is the perceived experience of sensory images and sounds during sleep, in a sequence in which the dreamer usually perceives more as an apparent participant than as an observer. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep.

As Dement studied, he found out that people need REM, or dreaming, sleep. He conducted a sleep and dream research project, in which the first eight of his participants were published in the article he wrote. All eight were male. For a span of a couple weeks, he varyingly deprived the participants of strictly REM sleep by waking them each time they started to enter the stage. He monitored this with small electrodes attached to their scalp and temples. As the study went on, he noticed that the more he deprived them of REM sleep, the more often he had to wake the men.

Dreams can also be suppressed or encouraged; taking anti-depressants, acetaminophen, ibuprofen, or alcohol is thought to potentially suppress dreams, whereas melatonin may have the ability to encourage them.