Stages of Sleep
Scientists used to think that sleep was simply a period of brain inactivity. Now researchers know that sleep is an active process for the brain. You spend about one-third of your life asleep, but it's not wasted time. This is the time the body uses to repair and restore itself.
One way that scientists measure sleep is by measuring electrical activity in the brain. Waves of electrical activity happen in the brain during sleep. Your brain waves change during the 3 basic stages of nonrapid eye movement (NREM) sleep. The NREM stages are:
N1. The first stage of sleep is the lightest stage. At this point, your body processes slow down, and you may have a feeling of falling. You begin drifting toward deeper stages of sleep. If awakened, which happens easily in this stage, you may remember images or dreams. Your arm, leg, or another part of your body may move suddenly in this stage.
N2. You spend about half of your slumber in this stage. Your eye movements stop and your brain waves slow down. If the electrical pulses that make up your brain waves were tracked, you would see slow waves known as theta waves and sudden bursts of activity, called sleep spindles. This is when bedroom temperature matters. A room that's too warm or too cold can make it hard to reach this stage of sleep.
N3. Deeper into sleep, your brain waves slow down even more, into delta waves. You still may have sudden bursts of brain activity at this stage. N3 is when your body and brain make all the repairs that help you recover after a tough day. This is your most restorative sleep. It's much harder to awaken from this stage than from N1. In fact, you could be very disoriented if this were to happen. Sleepwalking and sleep talking are most common in this stage. During N3, your body also releases hormones crucial to growth and development.
Rapid eye movement (REM)
Another stage of sleep is distinct from the other 3 stages. It's marked by rapid eye movement (REM). You spend about 20% to 25% of your sleep time in REM sleep. This is a deep sleep yet an active period for your brain. In contrast to adults, infants spend nearly half of their sleep in this stage. There is much more going on than brisk eye movement, including most of your dreaming. Most of your muscles don't move during this stage. Some are even temporarily paralyzed. This is a condition called atonia, which prevents you from acting out dreams. If awakened, you might remember vivid, lifelike dreams. Brain waves in this stage are often quite similar to those when you are awake.
REM sleep can occur several times throughout the night. The first REM phase typically occurs about 90 minutes after you fall asleep. You will start the night cycling between non-REM and REM sleep every 90 to 110 minutes or so. As the night continues, your sleep will become lighter (N1 and N2) in between REM cycles. You will lose some ability to manage your body's temperature at this stage. This is why people may wake up if the room is too hot or too cold.
Although scientists used to think that people did not dream during non-REM sleep, some complex mental activity has been recorded during these periods. In addition, adults who awoke from night terrors during non-REM sleep have also reported dream-like activity. So some researchers believe that we dream during these stages as well.