Sleep: What It Is, Why It’s Important, Stages, REM & NREM

Modern medicine’s understanding of sleep is like a partially assembled jigsaw puzzle. Experts can identify some of the pieces and have an idea of what the big picture looks like, but they haven’t figured out how everything fits together.

Your body cycles between being awake and asleep throughout each day, with certain processes only happening when you’re asleep. When you’re asleep, your body “powers down” and most body systems — including your brain — become less active.

Some of the key things that happen while you’re asleep include:

Sleep is variable from person to person. The way that people sleep and how much they need can vary widely. The number of hours of sleep that are good for your health can also change during your lifetime.

In general, recommended sleep amounts by age are:

These sleep amounts apply to most people, but they aren’t universal. Some people need more sleep, and others need less. Variations in how much sleep you need may even be genetic. For example, some people can inherit the trait of being a “short sleeper” from a parent.

Personal circumstances and your health status can also affect how much sleep you need. People who are sick or recovering from an injury or medical procedure may need to sleep more. Pregnant people often need more sleep during the first trimester, too.

If you have questions about how much sleep you need, especially if it’s different from the amount recommended for your age group, talk to a primary care provider. They can help you understand when that difference might mean there’s a medical issue that needs exploring.

No, getting all of your sleep at once isn’t a hard-and-fast rule. In fact, sleep customs commonly vary by culture and time period. Historically, some cultures embraced splitting nighttime sleep into two periods. Throughout the world, many cultures still embrace the habit of napping. Many have their own word for it (like “siesta” in Spanish-speaking countries or the “inemuri,” a short workplace nap that’s practiced in Japan).

But like too much of anything, napping comes with a drawback. Napping for too long can affect sleep quality overnight. There’s also an increased risk for certain health problems. Talk to your healthcare provider if you often nap or want to try it. That can allow you to get the most benefit from naps without the drawbacks.

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Sleeping doesn’t mean your brain is totally inactive. While you’re less aware of the world around you, you still have plenty of detectable brain activity. That brain activity has predictable patterns. Experts organized those patterns into stages. The stages fall broadly into two categories: rapid eye movement (REM) sleep and non-REM (NREM) sleep.

There are three NREM stages. When you fall asleep, you typically enter NREM stage 1 and then cycle between NREM stages 2 and 3. After that, you go into REM sleep and start dreaming. After the first REM cycle, you start a new sleep cycle and go back into stage 1 or 2, and the cycle starts over.

One cycle normally takes about 90 to 120 minutes before another begins. Most people go through four or five cycles per night (assuming they get a full eight hours of sleep).

Stage 1 NREM sleep is the lightest stage of sleep. You enter stage 1 right after you fall asleep. This stage usually lasts only a few minutes, making up about 5% of your sleep time. After that, your sleep gets deeper, and you move into stage 2 NREM sleep.

Stage 2 is still light sleep, but deeper than stage 1. During this stage, your brain waves slow down and have noticeable pauses between short, powerful bursts of electrical activity. Experts think those bursts are your brain organizing memories and information from the time you spent awake.

Stage 2 NREM sleep accounts for about 45% of your time asleep (the most of any stage). You’ll go through multiple rounds of stage 2 NREM sleep, and usually, each one is longer than the last. After stage 2, you move deeper into stage 3 NREM sleep or enter REM sleep.

The deepest stage of NREM sleep is stage 3. It makes up about 25% of your total sleep time in adults. But babies and children need more stage 3 sleep, and the older you get, the less you need.

In stage 3, your brain waves are slow but strong. Your body takes advantage of this very deep sleep stage to repair injuries and reinforce your immune system. The same bursts of brain activity that happen in stage 2 can also happen in stage 3, and brain waves specific to stage 3 help regulate those bursts.

You need stage 3 NREM sleep to wake up feeling rested. Without enough stage 3 sleep, you feel tired and drained even if you slept for a long time. That’s why your body automatically tries to get as much stage 3 sleep into your sleeping period as early as possible. After stage 3 NREM sleep, your body moves into stage 2 NREM, which is the gatekeeper of REM sleep.

Because stage 3 NREM sleep is so deep, it’s hard to wake someone up from it. If they do wake up, they’ll probably have “sleep inertia,” a state of confusion or “mental fog.” Sleep inertia lasts about 30 minutes.

Rapid eye movement (REM) sleep is the stage of sleep where most dreams happen. Its name comes from how your eyes move behind your eyelids while you’re dreaming. During REM sleep, your brain activity looks very similar to brain activity while you’re awake.

REM sleep makes up about 25% of your total time asleep. Your first REM cycle of a sleep period is typically the shortest, around 10 minutes. Each one that follows is longer than the last, up to an hour.