Our brains cycle through four types of brain waves, referred to as delta, theta, alpha and beta. Each type of brain wave represents a different speed of oscillating electrical voltages in the brain.
Delta is the slowest (zero to four cycles per second) and is present in deep sleep.
Theta (four to seven cycles per second) is present in stage one when we're in light sleep.
Alpha waves, operating at eight to 13 cycles per second, occur during REM sleep (as well as when we are awake).
And beta waves, which represent the fastest cycles at 13 to 40 per second, are usually only seen in very stressful situations or situations that require very strong mental concentration and focus.
These four brain waves make up the electroencephalogram (EEG).