Light sleep, deep sleep and REM: how they differ, and how accurately a watch can tell

You don't sleep the same way all night long. Your brain and body cycle through several stages of sleep, more than once. Each stage does something different, and sleep stages are among the numbers many wearables report every morning.

Two types of sleep

The US National Institute of Neurological Disorders and Stroke (NINDS, NIH) divides sleep into two main types: REM (rapid eye movement) and non-REM, which splits into three stages. Each stage has its own pattern of brain waves and nerve cell activity.

Non-REM Stage 1 and Stage 2

• Stage 1 is the switch from awake to asleep. It is a short spell of light sleep in which your heartbeat, breathing and eye movements slow down. It usually lasts just a few minutes.
• Stage 2 is light sleep before you enter deep sleep. Your body temperature drops and your eyes stop moving. NINDS notes that in each sleep cycle, we spend more time in Stage 2 than in any other stage.

Non-REM Stage 3, or deep sleep

Stage 3 is the deep sleep you need to wake up feeling refreshed. Your heart and breathing are at their slowest of the night, your muscles relax, and you may be hard to wake. NINDS notes that deep sleep tends to run longer in the first half of the night.

REM

REM first starts about 90 minutes after you fall asleep. Your eyes move quickly from side to side behind closed eyelids, brain wave activity becomes closer to that of being awake, and your breathing becomes faster and irregular. Most dreams happen in this stage, and your arms and legs briefly can't move, which keeps you from acting out your dreams. NINDS also notes that we spend less time in REM as we age, and that memory consolidation most likely needs both non-REM and REM sleep.

Several cycles a night

On a typical night, we cycle through non-REM and REM several times, and NINDS notes that REM periods become longer and deeper toward the end of the night.

How accurately can a watch measure sleep stages

The standard measurement is an overnight test in a sleep lab (polysomnography), which uses brain waves. A study by Chinoy et al. (Sleep, 2021) compared seven consumer devices with polysomnography and found that the devices were good at telling whether you were asleep or awake, but their sleep stage results were inconsistent. An editorial in the same journal concluded that sleep staging from these devices is a "rough estimate" and does not replace a sleep lab test.

How to read your own numbers

1. Don't take one night too seriously. A wearable's sleep stage numbers for a single night can be off. 2. Look at the trend over many nights. If deep sleep or REM keeps dropping for several nights in a row, check whether your bedtime, wake time or evening routine has changed. 3. If sleep problems keep coming back, such as feeling very sleepy during the day, loud snoring with pauses in breathing, or regularly being unable to fall asleep, talk to a doctor.

The Health tab in Align shows your sleep stages when you connect a supported wearable, so you can see the daily trend alongside your HRV and resting heart rate.

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*This article is general information, not medical advice. If you have sleep problems or health concerns, please talk to a doctor.*

Sources

• National Institute of Neurological Disorders and Stroke (NIH). Brain Basics: Understanding Sleep. https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep
• Chinoy ED, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep 2021;44(5):zsaa291.
• Grandner MA, Lujan MR, Ghani SB. Sleep-tracking technology in scientific research: looking to the future. Sleep 2021;44(5):zsab071. https://pmc.ncbi.nlm.nih.gov/articles/PMC8343587/