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You dream about two hours nightly
Sleep begins with three stages of non-rapid eye movement sleep, and REM sleep arrives about an hour and a half or two hours after falling asleep, depending on the person. REM wasn't discovered until the 1950s. Dreaming was once thought to happen only in REM, and it is now known to happen in both kinds of sleep. REM dreams are the most intense and the likeliest to be remembered.
Dreams occur multiple times a night and last a few minutes, maybe up to half an hour, which adds up to a couple of hours of dreaming. Nobody knows exactly how long they last, because a sleeper can't hit a timer when a dream starts. Researchers hook sleepers up to machines and infer when a dream is happening.
Why do dreams grow longer toward morning?
Sleep runs in cycles of roughly ninety minutes, each moving from light sleep to deep sleep and then to REM. Early cycles are dominated by deep slow-wave sleep, while later cycles hold less of it and more REM. The first REM period lasts only about ten minutes, and the one before waking can last close to an hour. This is why the dreams a person remembers on waking tend to come from the last hours of the night.
How was REM sleep discovered?
In 1953, Eugene Aserinsky, a graduate student working with Nathaniel Kleitman at the University of Chicago, noticed that sleeping people's eyes sometimes darted beneath closed lids. The recordings showed brain activity resembling wakefulness during these periods. When volunteers were woken then, they often reported vivid dreams. Aserinsky and Kleitman published the finding in the journal Science that year.
How do researchers tell someone is dreaming?
Sleep labs record brain waves, eye movements, and muscle tone at the same time, a method called polysomnography. REM shows up as fast, low-voltage brain waves, darting eyes, and muscles gone limp. Researchers can then wake the volunteer and ask what was going through their mind. Dream reports collected this way are the main evidence for how long and how often people dream.