REM Sleep

* Rapid Eye Movement sleep usually occurs about 90 minutes after falling asleep; it cycles along with non-REM stages throughout the night.

Non-REM Sleep

Stage 1:
Light sleep; easily awakened; muscle activity; eye movements slow down.

Stage 2:
Eye movements stop; slower brain waves, with occasional burst of rapid brain waves.

Stage 3:
Considered deep sleep; difficult to awaken; brain waves slow down more, but still have occasional rapid waves.

Stage 4:
Considered deep sleep; difficult to awaken; extremely slow brain waves. 

Source: National Institutes of Health, Bethesda, Md.