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.
Instead, our sleep is actually a continuous swirl of brain activity, with regular cycles of more intense activity alternating with less intense activity.
Human brains give off weak electrical signals called “brain waves” that can be measured outside the skull.
These signals reflect the combined activity of millions of neurons in particular regions of the brain.
When these signals are recorded and analyzed with an electroencephalograph (EEG), the characteristics of the waves correspond to distinct stages of consciousness and sleep.
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).
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
(but paralysis of most other muscles), intense brain activity, and vivid, active dreams.
The cycle of sleep stages occurs 3 to 5 times in a typical night's sleep.
A person awakened during REM sleep usually reports that a true dream was in progress, with vivid sensory hallucinations and imagined movements or actions.