Stage 1 sleep: As an individual becomes drowsy, and enters stage 1, alpha wave decreases and theta waves appear. Theta waves with a frequency of about 6 to 8 cycles per second are accompanied by slow rolling eye movements. Stage 1 is the lightest stage of sleep. After 30 to 40 minutes of stage 1 sleep one undergoes a rather steep descent into stages 2, 3 and 4.

Stage 2 sleep: In stage 2 sleep is composed of largely theta background, and is characterised by:

Sleep spindles: regular spindle shaped waves of 13-15 cycles/sec with waxing and waning amplitude

K-complexes- high voltage spikes present intermittently. Stage 2 occupies about 50% of sleep.

Stages 3 and 4, Slow wave sleep (SWS): During deep sleep stages 3 and 4, brain produces slow delta waves. In stage 3 delta waves have a frequency of 1 to 3 cycles per second.

Stage 4 sleep: The deepest stage of sleep, shows predominant delta activity with frequency of 0.5 to 2 cycles per second and highest amplitude. Slow wave sleep usually lasts for 70-90 minutes and takes place during the first hours of sleep. REM (rapid-eye-movement) SLEEP

After deep stage 4 sleep, comes REM sleep. It derives its name from the rapid eye movements; observable beneath the closed eyelids.REM sleep is characterised by rapid, low amplitude brain waves similar to that of light stage 1 sleep. REM sleep is associated with dream.