WEBVTT

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As you could see from this simulated sleep experiment, the brain does not shut down during the one-third of our lives that we spend asleep. 

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Instead, our sleep is actually a continuous swirl of brain activity, with regular cycles of more intense activity alternating with less intense activity. 

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Human brains give off weak electrical signals called “brain waves” that can be measured outside the skull. 

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These signals reflect the combined activity of millions of neurons in particular regions of the brain. 

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When these signals are recorded and analyzed with an electroencephalograph (EEG), the characteristics of the waves correspond to distinct stages of consciousness and sleep. 

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As a person drifts off to sleep, the EEG waves become gradually slower and larger as the person moves from NREM-1 (transition from wakefulness) to NREM-3 (deep sleep). 

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After a short time in NREM-3, the person moves back through the sleep stages, and, instead of reaching NREM-1, enters REM sleep, a period of rapid eye movements 

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(but paralysis of most other muscles), intense brain activity, and vivid, active dreams. 

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The cycle of sleep stages occurs 3 to 5 times in a typical night's sleep. 

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A person awakened during REM sleep usually reports that a true dream was in progress, with vivid sensory hallucinations and imagined movements or actions.