Why Your Brain Cycles Through Sleep Stages Every Night

Right now, while you're sound asleep tonight, your brain will travel through four totally different stages — some so deep your body barely twitches, and one so active that your eyes dart back and forth even with your eyelids shut. Scientists call this nightly trip the sleep cycle. It takes about 90 minutes to complete, and your brain repeats it 4 to 6 times before your alarm goes off, which is one reason kids who go to bed late but still have to wake up early feel so groggy — they get cut off in the middle of a cycle.

- Name all four stages of sleep in the order your brain moves through them - Explain what NREM stands for and what happens in Stages 1, 2, and 3 - Describe what makes REM sleep different from every other stage - Explain why a full night's sleep means finishing several complete cycles, not just closing your eyes

Stage 1 is the shortest stage of all — it usually lasts only 1 to 5 minutes. Your muscles relax, your breathing slows, and you're still so close to being awake that a tiny noise can pull you right back out. Some people even feel a sudden muscle twitch called a hypnic jerk, like your body thinks it's falling. Then you slide into Stage 2, which makes up almost half of your total sleep time. Your heart rate slows, your body temperature drops slightly, and your brain starts producing short bursts of electrical activity called sleep spindles. Scientists who study brain waves with a machine called an EEG can actually see these spindles as sharp little spikes on the recording.

Stage 3 is often called slow-wave sleep because your brain waves slow way down into big, rolling patterns called delta waves. This is the deepest stage of sleep — if someone tries to wake you up now, you'll feel confused and foggy for a few minutes. Stage 3 is also when your body does its repair work. Your pituitary gland releases growth hormone, which helps rebuild muscle tissue and repair cells damaged during the day. This is also when sleepwalking and night terrors are most likely to happen in kids, because the brain is so deeply asleep that it has trouble fully waking up even when the body starts moving.

About 90 minutes after you fall asleep, you enter REM sleep — short for Rapid Eye Movement. The first REM stage of the night lasts only about 10 minutes, but each cycle's REM stage gets longer, stretching to nearly an hour by early morning. During REM, your eyes dart back and forth rapidly beneath your closed eyelids, and an EEG recording of your brain looks almost identical to a brain that's wide awake. But your arms and legs go essentially paralyzed — a state called muscle atonia — which is your body's safety switch to stop you from acting out your dreams. REM is also when your brain sorts and files memories from the day. In sleep-lab studies, students who slept after learning new material remembered significantly more of it the next day than students who stayed awake.