Physiology. Periods of sleep manifested by changes in EEG activity and certain behavioural correlates. There are two different kinds of sleep: NREM sleep and paradox sleep stage (REMS). The NREM sleep includes stage 1: sleep onset, drowsy sleep; stage 2: light sleep; stage 3: intermediary sleep and 4: deep sleep. The stage of REM (rapid eye movement) sleep in which brain activity is extensive, brain metabolism is increased, and dreaming occurs.