Also known as "dream sleep," REM (rapid eye movement) is the stage of sleep when your brain processes your thoughts and memories from the day.
While you are sleeping, your brain moves through a series of sleep cycles that repeat themselves about every hour and a half, or five times, throughout the night. Each cycle consists of five distinct stages of sleep. The first four of these are called non-rapid eye movement (non-REM) stages, and the last stage of each cycle is known as rapid-eye-movement (REM) sleep.
As the night (and your sleep) progresses, you move more quickly from the first two stages of sleep to REM sleep, so that by morning you skip stages three and four and go directly to REM sleep. That means REM periods of sleep get longer and longer as you get closer to the morning and the time you normally wake up. Over the course of six to eight hours during a normal night, you get about four to six hours of non-REM sleep and about two hours of REM sleep.
During REM sleep, your eyes are darting back and forth rapidly under closed lids, hence the name of this stage. This eye movement, along with your brain activity is similar to what it is when you are awake. At the same time, you are temporarily paralyzed during REM sleep. So your mind is active but your body is still. This is what prevents you from physically acting out your dreams. If this safety mechanism wasn't in place, as with some REM sleep disorders, you might hurt yourself or others while acting out violent dreams.
Although you dream during both REM and non-REM sleep, your dreams are usually deeper and more vivid and story-like during the REM stages and during non-REM stages three and four, that lead into REM sleep. You are more likely to remember those dreams than any you have during earlier stages of non-REM sleep.
Although experts have some idea what happens during each stage of sleep, no one knows exactly why these different stages exist, just as no one knows for sure exactly why we need any sleep at all. But neuroscientists who study sleep patterns think that one important function of sleep is to convert the thoughts and feelings you experience throughout the day into permanent memories. A University of California study published in a 2009 issue of the professional journal Cerebral Cortex confirmed that this is more likely to happen during REM sleep than during other stages.
While in the REM stage, you are in the deepest period of sleep, which makes it more difficult to wake up than when you are in one of the non-REMstages. REM sleep is not considered better quality sleep than non-REM sleep, however, because all of these stages work together to provide you with a restful night's sleep.
When you were a baby, you slept about 16 hours every day, and half your sleep time was spent in the REM stage. As an adult, that changes and the amount of time you spend in REM sleep drops to about 20 percent. The older you get, the less time you spend in REM sleep, until it drops closer to only about 14 or 15 percent of your total sleep time, or less than an hour. If your sleep is interrupted during the night, or if you have a rare sleep disorder that affects the REM stages of sleep, you may not spend enough time in this stage. But if you wake up feeling refreshed and are able to perform your daily tasks normally and successfully throughout each day, you are probably getting enough REM sleep.
Sources:
Nishida, M. et al. "REM Sleep Prefrontal Theta, and the Consolidation of Human Emotional Memory." Cerebral Cortex. 2009 May; 19(5):1158-1166. Web. 29 July 2011
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2665156/
