If you attach an electroencephalograph to a person's head, you can record the person's brainwave activity. An awake and relaxed person generates alpha waves, which are consistent oscillations at about 10 cycles per second. An alert person generates beta waves, which are about twice as fast.

During sleep, two slower patterns called theta waves and delta waves take over. Theta waves have oscillations in the 3.5 to 7 cycle per second range and delta waves are those below 3.5 cycles per second. As a person falls asleep and sleep deepens, the brainwave patterns slow down. The slower the brain wave patterns, the deeper the sleep - a person deep in delta wave sleep is hardest to wake up.

At several points during the night, Rapid Eye Movement (REM) sleep occurs, and brainwaves during this period speed up to awake levels (alpha or beta).

Further reading:
1. Sleep by J. Allan Hobson
2. How Sleep Works by Marshall Brain


"What difference is there between us, save a restless dream that follows my soul but fears to come near you?"
Kahlil Gibran



Different Stages of Sleep
REM Sleep
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