There are two phases of sleep whose names depend on whether or not the eyes can be seen to move behind the closed eyelids: NREM
(nonrapid eye movement) and REM
(rapid eye movement) sleep. The electroencephalography (EEG) waves during NREM sleep are of high amplitude and low — that is, slow — frequency, and NREM sleep is also referred to as slow-wave sleep.
REM sleep is also called paradoxical sleep
because the sleeper is difficult to arouse despite having an EEG that is characteristic of the alert, awake state. In fact, brain O2 consumption is higher during REM sleep than during the NREM or awake states. When awakened during REM sleep, subjects generally report that they have been dreaming. This is true even in people who do not remember dreaming when they awaken on their own.
We still know very little about the function of sleep. Non-REM sleep seems to have a restorative effect on the body, and
prolonged sleep deprivation is fatal to experimental animals. Yet it is unclear why quiet bed rest is not sufficient—that is, why we must lose consciousness. Unlike total sleep deprivation, selective deprivation of REM sleep (waking a person up whenever he or she begins REM sleep) has no adverse effects. Some have suggested that REM sleep is a period in which the brain either “consolidates” and strengthens memories or purges unwanted information from memory, but there is little evidence for either view.
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