Sleep: periodic, natural loss of consciousness—as distinct from unconsciousness resulting from a coma, general anesthesia, or hibernation.
REM sleep: rapid eye movement sleep; a recurring sleep stage during which vivid dreams commonly occur. Also known as paradoxical sleep,because the muscles are relaxed (except for minor twitches) but other body systems are active
Dream: a sequence of images, emotions, and thoughts passing through a sleeping person’s mind. Dreams are notable for their hallucinatory imagery, discontinuities, and incongruities, and for the dreamer’s delusional acceptance of the content and later difficulties remembering it.
Sigmund Freud theorized about the supposed latent content (hidden meaning) found in dreams. According to him, a dream's manifest content (literal meaning) is a window to see deeper, more symbolic desires in a person's subconscious. His theories surrounding dreams have been mostly disproved by modern psychology, in which most believe that dreams hide nothing.
In one of his 15,000 research participants, William Dement (1999) observed the moment the brain’s perceptual window to the outside world slammed shut. Dement asked this sleep-deprived young man, lying on his back with eyelids taped open, to press a button every time a strobe light flashed in his eyes (about every 6 seconds). After a few minutes the young man missed one. Asked why, he said, “Because there was no flash.” But there was a flash. He missed it because (as his brain activity revealed) he had fallen asleep for 2 seconds. Unaware that he had done so, he had missed not only the flash 6 inches from his nose but also the abrupt moment of his entry into sleep.
William Dement was able to isolate the change in brain activity that occurs when a person falls asleep by asking a sleep-deprived man to push a button every time a strobe light flashed before his eyes. Eventually he missed a flash when he fell asleep for two seconds, and Dement was able to determine the corresponding brain activity.
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