Sleep stages (REM, deep, light)
The four sleep stages explained, what each does for your body and how wearables track them.
Your body repairs itself in layers
Every night you cycle through four distinct stages, each with a specific job. Think of it like a maintenance crew working in shifts: one team handles the body, another the brain, and they rotate every 90 minutes or so. A full night holds four to six of those cycles, and they are not identical: the first cycles carry most of the deep sleep, the last ones most of the REM.
Light Sleep (N1 + N2), about half your night
This is where sleep begins. Your muscles relax, your heart slows down, and your brain starts producing sleep spindles, short bursts of activity that help lock in memories from the day. N1 is the doorway, a few minutes of drifting that you would deny was sleep if woken from it; N2 is the bulk. Light sleep makes up roughly 50 to 60 % of total sleep time, and "light" does not mean useless: N2 is where motor learning consolidates.
Deep Sleep (N3), the physical repair shop
Deep sleep is when the real restoration happens. Your body releases growth hormone, repairs damaged tissues, and strengthens the immune system; the brain's glymphatic system clears metabolic waste most actively in this stage. It's the hardest stage to wake from, and unfortunately it naturally declines as you age, from around 20 % of the night in your twenties toward 10 % or less past 60 (Ohayon 2004). Typically 15 to 25 % of your night as an adult.
REM Sleep, the brain's filing system
REM is where most dreaming takes place. Your brain processes emotions, consolidates complex memories, and clears out mental clutter, while the body is temporarily paralysed so that dreams stay dreams. REM periods get longer toward morning, which is one reason cutting sleep short is so costly for cognition: the last hour of a full night is mostly REM. About 20 to 25 % of the night.
Brief awakenings
Waking up briefly between cycles is completely normal; most people don't even remember it. Wearables track these moments to calculate your sleep efficiency.
How wearables see stages
A sleep lab reads stages from brain waves, eye movement and muscle tone. A watch has none of those. It infers stages from wrist movement, heart rate and heart-rate variability, using the fact that deep sleep is still with a low, steady pulse, REM is still with a variable pulse and slightly raised heart rate, and light sleep sits in between. Against polysomnography, consumer devices get the sleep-versus-wake decision right about 90 % of the time but agree with the lab on the exact stage only about half to two thirds of the time, with typical errors of 20 to 30 minutes per stage per night (Chinoy 2021; de Zambotti 2019). Deep and REM are the stages most often confused with each other.
What follows from that: your device's stage split is indicative, its night-to-night trend on the same device is meaningful, and a comparison of "deep sleep percentage" between two brands is meaningless, because each brand's algorithm draws the boundaries differently.
What changes the mix
- Alcohol: suppresses REM in the first half of the night, then fragments the second half as it wears off
- Late bedtime: deep sleep is anchored to the early night and the homeostatic sleep drive, REM to the circadian morning. Go to bed very late and you keep the deep sleep but cut the REM; wake very early and you lose the REM-rich last cycle
- Hard training: increases deep sleep on the following night, provided it did not end within two or three hours of bedtime
- Heat: a warm bedroom reduces deep sleep; core temperature has to fall for N3
- Age: deep sleep shrinks, awakenings grow, REM holds roughly steady until late life
- Some medicines: many antidepressants suppress REM; benzodiazepines and Z-drugs increase light sleep at the expense of deep
How MyBodyAI uses sleep stages
Your sleep phase data flows into Sleep Quality, Sleep Debt, Recovery, Immunity, Body Alert and the overtraining and respiratory indices. What is scored is the share of the night spent in deep sleep, in REM and awake; the deep-sleep target is adjusted for your age so that a 60-year-old is not scored against a 25-year-old's night.
Tracked by Polar, Fitbit, Withings and Oura with a connected account, and by Garmin from your uploaded data export.
Frequently asked questions
How much deep sleep should I get?
Adults typically spend 15 to 25 % of the night in deep sleep, about 1 to 2 hours of an 8-hour night, and the share falls with age, from around 20 % in your twenties to 10 % or less after 60. Because a watch estimates stages from pulse and movement, treat its deep-sleep minutes as approximate and compare nights on the same device rather than against a fixed target.
Are sleep stages from a smartwatch accurate?
Partly. Devices tell sleep from wake correctly about 90 % of the time, but match a sleep lab on the exact stage only about half to two thirds of the time, with errors of 20 to 30 minutes per stage. Deep and REM are the most often confused. The trend across nights is useful; a single night's stage split is not something to act on.
Why is my REM sleep low?
The usual causes are alcohol, which suppresses REM in the first half of the night, a short night, because REM is concentrated in the last cycles, and some medicines, especially many antidepressants. A single low night after a late dinner with wine is expected; a low share every night for weeks with enough total sleep is worth a conversation with a doctor.