The Stages of Sleep: How Sleep Works and How to Sleep Better

Every night you cycle through four stages of sleep, each with a different job: light sleep eases you in, deep sleep handles physical repair, and REM sleep works on memory and mood. When those cycles run on a steady schedule and get enough hours, you recover better, think more clearly and train harder.

This guide explains the stages of sleep, your body clock, how much sleep you need, how sleep affects training and recovery, and what actually helps you fall asleep faster, from naps and caffeine to supplements, travel and sleep trackers.

The short version

• Sleep runs in cycles of roughly 90 minutes, four to six times a night. Each cycle moves through three stages of non-REM sleep and then REM sleep.
• Deep sleep is concentrated early in the night and REM sleep late, so cutting your night short mostly costs you REM.
• Adults need at least 7 hours a night. Regular short sleep hurts training, recovery, appetite and mood.
• The habits with the best track record are simple: a consistent wake time, morning light, a cool, dark bedroom, an early caffeine cutoff and a wind-down routine.
• Most supplements have weak evidence. Melatonin is most useful for jet lag and shifting your body clock, not for everyday insomnia.

The stages of sleep

Sleep has two main types: non-REM sleep, which has three stages, and REM (rapid eye movement) sleep. You move through them in order, from light sleep into deep sleep and then into REM, and then the cycle starts again. A full cycle takes roughly 90 minutes, and most people complete four to six cycles a night.

The mix changes through the night. Early cycles hold most of your deep sleep, while later cycles hold longer stretches of REM. That is why a short night costs you a large share of your REM sleep. Sleeping in on weekends does not fully make up for it either: in one NIH-funded study, weekend catch-up sleep did not undo the effects of weekday sleep loss on late-night snacking and insulin sensitivity.

You may still see “stage 4” in older articles. Sleep scientists merged stages 3 and 4 into a single deep sleep stage, N3, in 2007. Deep sleep also declines with age, which is one reason older adults wake more easily during the night.

Your body clock

Two systems decide when you feel sleepy. The first is your circadian rhythm, a roughly 24-hour clock run by a small region of the brain called the suprachiasmatic nucleus. The second is sleep pressure: a chemical called adenosine builds up the longer you are awake, and sleep clears it.

Light is the main signal that sets your body clock. Bright light in the morning anchors it, while darkness in the evening lets your body release melatonin, the hormone that signals night. Caffeine works by blocking adenosine, which is why it can hide sleepiness without removing your need for sleep.

People also differ in timing. Some are natural early risers and others are night owls, and teenagers tend to shift later. Your clock can be nudged, but it responds best to consistency: the same wake time every day, including weekends.

How much sleep you need

These are the CDC’s recommendations, which draw on guidance from the American Academy of Sleep Medicine and the National Sleep Foundation:

Regularly sleeping less than you need adds up. Reaction time, focus and mood suffer first. Appetite changes too: in a review of controlled studies, people whose sleep was cut short ate roughly 385 more calories a day, without burning any more, which makes weight management harder. Over time, short sleep is also linked to higher risks of high blood pressure, type 2 diabetes and depression.

Sleep and mental health work in both directions. Stress, anxiety and low mood make it harder to sleep, and poor sleep makes them harder to manage. Improving one often helps the other.

Sleep, training and recovery

Sleep is when much of your training pays off. The largest pulse of growth hormone comes during deep sleep early in the night, and your body uses sleep to repair muscle and restock energy.

Studies point the same way:

• Muscle repair: in one study, a single night without sleep cut muscle protein synthesis by 18% the next day.
• Performance: when Stanford basketball players extended their sleep for several weeks, their sprint times got faster and their shooting accuracy rose by about 9%.
• Injuries: in a study of adolescent athletes, those who slept less than 8 hours a night were about 1.7 times more likely to be injured.

A few practical rules follow. Treat sleep as part of your training plan, not what is left over. After a bad night, keep your session but lower the intensity. If you can choose, put your hardest workouts after good nights.

Evening exercise is fine for most people. Research suggests a moderate workout in the evening does not hurt sleep, though a very hard session in the last hour before bed can make it harder to fall asleep. Gentle stretching and mobility work before bed can help you unwind; see our best exercises before bed.

How to fall asleep faster and sleep better

None of these habits works overnight, but together they have the strongest evidence behind them. Start with the first two.

Keep a steady wake time and get morning light

Wake up at the same time every day, even after a late night. Then get outside or near a bright window for 15 to 30 minutes in the morning. Together these anchor your body clock, so you feel sleepy at a consistent time in the evening. If you need to shift your schedule, move it by 15 to 30 minutes every few days rather than all at once.

Make the bedroom cool, dark and quiet

• Cool: around 65 degrees Fahrenheit (18 degrees Celsius) suits most people. Your body temperature needs to drop to fall asleep.
• Dark: blackout curtains or a comfortable sleep mask block streetlights and early sunrises, which is especially useful in a city apartment.
• Quiet: earplugs or a white noise machine can mask traffic, sirens and neighbors.
• Comfortable: a supportive mattress and the right pillow matter more than most gadgets. See our guide to choosing the right pillow.

Time your caffeine, alcohol, meals and fluids

• Caffeine: caffeine has a half-life of about 5 hours, and in one study a large dose (400 mg) taken 6 hours before bed still cut total sleep by more than an hour. A good rule is no caffeine within 8 hours of bedtime, longer for strong drinks or pre-workout supplements, which for most people means nothing after early afternoon.
• Alcohol: a drink may help you fall asleep, but it fragments sleep in the second half of the night and cuts into REM. Keep it light and early.
• Meals: finish dinner two to three hours before bed. A small snack is fine if you are hungry.
• Fluids: drink most of your water earlier in the day and taper off in the last hour or two, so you are not up at night for the bathroom.

Put screens away

Screens affect sleep in two ways: their light can delay melatonin, and the content keeps your brain engaged. Research suggests the light from a phone has only a modest effect, so what you are doing on it, and how late it keeps you up, may matter as much or more. Night mode has not been shown to improve sleep much, so the bigger win is putting your phone away 30 to 60 minutes before bed and charging it outside the bedroom if you can.

Build a wind-down routine

Give yourself 30 to 60 minutes to shift gears: dim the lights, take a warm shower, read something on paper, or try a relaxation technique.

• Slow breathing: breathe in through your nose for a count of 4 and out slowly for a count of 6 to 8, for a few minutes. Longer exhales help calm your nervous system.
• Progressive muscle relaxation: tense each muscle group for about 5 seconds, then release, working from your feet up to your face.
• Gentle yoga: a few minutes in poses such as child’s pose, reclined butterfly and legs up the wall. Small studies suggest regular yoga can improve sleep quality. A private instructor can build a routine for you; see our private yoga and Pilates.

If you cannot fall asleep

If you have been awake in bed for about 20 minutes, get up, go to another room with dim light and do something calm until you feel sleepy, then go back to bed. Lying in bed frustrated teaches your brain that bed is a place for being awake. This technique comes from cognitive behavioral therapy for insomnia (CBT-I), the first-choice treatment for chronic insomnia. If worries keep you up, write them down earlier in the evening with one next step for each. And turn the clock around, because checking the time adds pressure.

Naps and coffee naps

A short nap can restore alertness without hurting your night. Keep it to 10 to 20 minutes and take it in the early afternoon, ideally before 3 pm. Longer naps are more likely to reach deep sleep, which leaves you groggy when you wake (this is called sleep inertia) and can make it harder to sleep at night. If you have insomnia, skipping naps usually helps.

A coffee nap combines the two: drink a cup of coffee, then nap right away for 15 to 20 minutes. Caffeine takes roughly 20 to 30 minutes to kick in, so it arrives as you wake up. Small studies suggest the combination beats either one alone for alertness, but it is a tool for an afternoon slump, not a substitute for sleep.

Food, drinks and supplements

No single food fixes sleep. A balanced diet with regular meal times helps more than any one ingredient. For supplements, the evidence is mostly modest:

Supplements are not approved for safety or effectiveness before they are sold, so quality varies. Choose products with third-party testing and check with your doctor or pharmacist, especially if you take prescription medication.

Travel, time zones and shift work

Anything that pulls your schedule away from daylight upsets your body clock.

• Jet lag: your body clock adjusts by roughly an hour a day, and flying east is usually harder than flying west. For a few days before the trip, shift your bedtime toward the new time zone. After arrival, get daylight at the right time (morning light after flying east, afternoon and evening light after flying west), eat on local time, and if you must nap, keep it under 30 minutes. Melatonin taken at the local bedtime can help you adjust after flying east.
• Red-eye flights: pick a window seat, bring an eye mask, earplugs and a neck pillow, skip alcohol, and drink water. Then stay up until a normal local bedtime after you land.
• Daylight saving time: the spring change costs most people an hour of sleep. In the few nights before, go to bed 15 to 20 minutes earlier each night, and get bright light the morning after the change.
• Shift work: keep your sleep times as consistent as you can, nap before a night shift, use blackout curtains for daytime sleep, and wear sunglasses on the way home. If you are exhausted or cannot sleep despite these steps, talk to your doctor about shift work disorder.

Sleep trackers and smart rings

Wearables such as smart rings and watches are good at measuring how long you sleep, your heart rate and your trends over weeks. They are much less accurate at telling sleep stages apart than a sleep lab test, so a single night’s “deep sleep” number should not worry you.

Use a tracker to see patterns, such as how late meals, alcohol or evening training change your sleep. If checking your score makes you anxious about sleep, take a break from it; sleep researchers coined a name for the problem in 2017: orthosomnia. For apps worth trying, see our best sleep apps.

When to talk to a doctor

Good habits fix a lot, but not everything. See your doctor or a sleep specialist if any of these sound familiar:

• Insomnia that will not go away: trouble falling or staying asleep at least three nights a week for three months or more. CBT-I is the recommended first treatment, ahead of sleeping pills.
• Signs of sleep apnea: loud snoring, gasping or pauses in breathing, waking with a dry mouth or headache, or feeling sleepy during the day despite enough time in bed. Sleep apnea is common and treatable.
• Constant mouth breathing at night: it can point to nasal blockage or sleep apnea. Get checked before trying mouth tape, which has little evidence behind it.
• Restless legs: an urge to move your legs in the evening that eases when you move. Low iron is linked to it, and a simple blood test can check your levels.
• Low mood, anxiety or racing thoughts: if worry or sadness keeps you up most nights, talk to your doctor or a mental health professional. Treating either problem often helps the other.

Frequently asked questions

How many sleep cycles do you need a night?

Most adults complete four to six cycles of about 90 minutes each, roughly 6 to 9 hours. Aim for at least 7 hours in total rather than an exact cycle count.

How much deep sleep do I need?

There is no single target, and it falls with age. Deep sleep usually makes up roughly 10 to 25% of the night. Getting enough total sleep on a steady schedule is the most reliable way to get enough of it.

Why do I wake up groggy?

Usually because an alarm or a long nap woke you from deep sleep, or because you are short on sleep overall. A consistent wake time and enough hours help most.

Is 6 hours of sleep enough?

For most adults, no. The recommendation is 7 or more hours, and people who regularly get 6 tend to underestimate how much it affects their focus and performance.

Is it OK to work out at night?

Yes for most people. A moderate evening workout does not harm sleep. Finish a very hard session at least an hour before bed.

Should I take melatonin to sleep?

It helps most for jet lag and for shifting your body clock. For ongoing insomnia, sleep experts recommend CBT-I instead. Talk to your doctor before using it regularly.

What is the best temperature for sleep?

Around 65 degrees Fahrenheit (18 degrees Celsius) works for most people, with a few degrees either way depending on your bedding.

Do power naps work?

Yes. A 10 to 20 minute nap in the early afternoon improves alertness without leaving you groggy or hurting your night.

Better sleep, better training

Sleep, training and nutrition work as one system. A Type A Training trainer can schedule your sessions and set their intensity around your sleep and recovery, and teach you a short evening mobility routine to help you wind down. Learn how in-home personal training works in Manhattan and parts of Brooklyn and Queens, or see our lifestyle coaching program, which brings training, nutrition and accountability together.