The sleep cycle: A sleep cycle lasts about 90 minutes and during that time we move through five stages of sleep. The first four stages make up our non-rapid eye movement (NREM) sleep, and the fifth stage is when rapid eye movement (REM) sleep occurs. NREM sleep: Across these four stages we move from very light sleep during Stage 1 down to very deep sleep in Stage 4. It is very difficult to wake someone who is in Stage 4 sleep. Across NREM sleep, we have little muscle activity and our eyes do not typically move, but all of our muscles retain their ability to function. REM sleep: As the name would imply – during this final stage of sleep, we have bursts of rapid eye movements. This is the stage of sleep in which most dreaming occurs. Our eyes are not constantly moving, but they do dart back and forth, up and down. These eye movements may be related to visual images of dreams, but that is not confirmed, and in general, the reason for these eye movements is still a mystery. Although our eyes are moving rapidly, the muscles that move our bodies are paralyzed (other important muscles, such as our heart and diaphragm continue to function normally).