Stages of Sleep
Sleep basically has two phases, NREM (Non Rapid Eye Movement) sleep and a REM (
It is difficult to wake a person who is in the REM phase of sleep while a person in the stage 3 and 4 (slow wave sleep) of the NREM phase of sleep, is also considered to be in deep slumber.
- Huxley, Aldous. In Coren S. Sleep Thieves: An Eye-Opening Exploration into the Science and Mysteries of Sleep. New York: The Free Press; 1996, p. 175.