Sleep Stages FAQ: REM, Deep Sleep and Core Sleep Explained

There are two types of sleep: rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep has three stages, so there are four stages in all. The National Heart, Lung, and Blood Institute (NHLBI) describes them this way:

1. Stage 1: the short changeover from being awake to being asleep.
2. Stage 2: light sleep. You are now asleep, and your body temperature drops.
3. Stage 3: deep sleep, also called slow-wave sleep.
4. REM sleep: your eyes twitch, your brain is active and most dreaming happens.

One full cycle takes about 80 to 100 minutes, according to the NHLBI. Most people go through four to six cycles a night, and you may wake briefly between them. Our Sleep Cycle Calculator uses a 90-minute midpoint to suggest bedtimes.

Yes. The mix shifts as the night goes on. Early cycles hold more deep sleep, and later cycles hold more REM sleep. REM periods get longer and deeper toward morning. That's according to the National Institute of Neurological Disorders and Stroke (NINDS). So when a night is cut short, much of the lost sleep is REM.

Deep sleep is stage 3 of non-REM sleep, also called slow-wave sleep. NINDS calls it the deep sleep you need to feel refreshed in the morning. Your heartbeat and breathing slow to their lowest levels. Your muscles relax, and you can be hard to wake. It happens mostly in the first half of the night.

There's no official nightly target for deep sleep. The AASM, SRS and NIH guidance is set in total hours, at 7 or more for adults. Deep sleep comes mostly early in the night, and the amount drops naturally with age. So aim for enough total sleep on a steady schedule. Don't chase a set number of deep-sleep minutes.

Deep sleep naturally falls across a lifetime. The NHLBI says slow-wave sleep peaks in early childhood and drops sharply in the teenage years. It keeps decreasing through adulthood, and some older people may have none at all. A tracker showing little deep sleep at 70 doesn't mean something is wrong.

REM stands for rapid eye movement. During REM sleep your eyes move quickly behind closed lids. Your brain activity looks close to its waking pattern, according to NINDS. Breathing turns faster and less regular, and heart rate and blood pressure rise toward daytime levels. The first REM period starts about 90 minutes after you fall asleep.

Like deep sleep, REM has no official nightly target. What NIH sources do describe is the pattern. REM periods get longer later in the night, so a short night can lose much of it. NINDS adds that people spend less time in REM as they age. The NHLBI notes you get less REM in cold rooms. Your body doesn't regulate its temperature well during REM.

Most dreaming happens during REM sleep, though some happens in non-REM sleep too. NINDS says you spend about two hours dreaming each night. You may not remember most of it. Dreams are usually most vivid in REM. Their exact purpose isn't known, though dreaming may help you process emotions.

Your arm and leg muscles become temporarily paralyzed during REM sleep. NINDS explains that this stops you from acting out your dreams. The NHLBI describes the same effect as muscles going limp. It's a normal part of healthy sleep.

"Core" is the Apple Watch Sleep app's name for stages 1 and 2 of non-REM sleep. Apple's validation paper compares Core to what other trackers call "light sleep." Apple chose a new word on purpose. Stage 2 is a normal, important part of sleep, and "light" can sound like a problem.

Light sleep covers stage 1 and stage 2 of non-REM sleep. In stage 1, your heartbeat, breathing and eye movements slow, and it usually lasts only several minutes. In stage 2, your body temperature drops and eye movements stop. NINDS says you spend more of your sleep cycles in stage 2 than in any other stage.

No. A large share of light sleep is normal. Apple's validation paper notes that stage 2 often makes up more than 50% of a night's sleep. So a tracker showing mostly core sleep is describing a typical night, not a problem.

The gold standard is polysomnography (PSG), an overnight sleep study. Sensors record brain waves, heart rhythm, breathing, oxygen levels and leg movements. Trained technologists then score each 30-second slice of the night by stage. Consumer devices estimate stages instead of measuring brain waves directly.

A smartwatch can estimate them, not measure them like a sleep lab. Apple says its watch classifies each 30 seconds of sleep using motion data, including tiny breathing movements. Apple tested it against lab and home sleep studies. It used 1,171 nights to build the model and 299 more to check it. NINDS describes consumer sleep tech as a way to informally collect sleep data.

Use them for trends, not diagnoses. Here are three practical habits:

• Watch the weekly pattern. One odd night tells you little.
• Focus on total sleep first. It's the number the guidelines are built on.
• Pair data with how you feel. Waking rested counts for more than a stage score.

If you feel unrested despite good numbers, see a clinician. Only a sleep study can diagnose a sleep disorder.