Types of Sleep

REM Sleep

Usually first occurs about 90 minutes after you fall asleep; cycles along with the non-REM stages throughout the night.
Eyes move rapidly, with eyelids closed.
Breathing is more rapid, irregular, and shallow.
Heart rate and blood pressure increase. Dreaming occurs.
Arm and leg muscles are temporarily paralyzed.

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 bursts 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.

The first period of REM sleep you experience usually occurs about an hour to an hour and a half after falling asleep. After that, the sleep stages repeat themselves continuously while you sleep. As the night progresses, REM sleep time becomes longer, while time spent in non-REM sleep stages 3 and 4 becomes shorter. By morning, nearly all your sleep time is spent in stages 1 and 2 of non-REM sleep and in REM sleep. If REM sleep is disrupted during one night, REM sleep time is typically longer than normal in subsequent nights until you catch up. Overall, almost one-half your total sleep time is spent in stages 1 and 2 non-REM sleep and about one-fifth each in deep sleep (stages 3 and 4 of non-REM sleep) and REM sleep. In contrast, infants spend half or more of their total sleep time in REM sleep. Gradually, as they mature, the percentage of total sleep time they spend in REM progressively decreases to reach the one-fifth level typical of later childhood and adulthood.

Why people dream and why REM sleep is so important are not well understood. It is known that REM sleep stimulates the brain regions used in learning and the laying down of memories. Animal studies suggest that dreams may reflect the brain's sorting and selectively storing important new information acquired during wake time. While this information is processed, the brain might revisit scenes from the day while pulling up older memories. This process may explain why childhood memories can be interspersed with more recent events during dreams. Studies show, however, that other stages of sleep besides REM are also needed to form the pathways in the brain that enable us to learn and remember.