Sleep Stages Explained: Light, Deep and REM Sleep

How to read a stage chart without over-reading it

A sleep report usually shows a chart of stages across the night, sometimes called a hypnogram, along with a percentage for each stage. Two things are worth keeping in mind. First, the National Heart, Lung, and Blood Institute notes that most nights include four to six cycles and that deep sleep tends to come earlier in the night while REM tends to come later, so the shape of the chart is expected to be lopsided and is not a sign that something is wrong.

Second, a consumer device is estimating the stages rather than measuring them. A single night’s chart therefore combines the device’s estimation error with ordinary night-to-night variation, which is why a pattern across several weeks says more than any one morning’s report. There is no clinical guideline that gives a target percentage for each stage, so a published "ideal" split on an app or website is a convention and not a standard.

When the stages are not the point

Understanding stages is useful background, but it is not a route to diagnosing a sleep problem. The symptoms that warrant medical attention are things a person notices rather than things a chart reveals.

• Loud snoring with gasping, choking or pauses in breathing noticed by a partner, which are characteristic of obstructive sleep apnea and need a validated test.
• Persistent daytime sleepiness despite spending enough hours in bed.
• Acting out dreams, shouting or moving violently during sleep, which can indicate a REM sleep behavior disorder and should be assessed by a clinician.
• Difficulty falling or staying asleep on most nights for three months or longer.
• Unrefreshing sleep alongside low mood or anxiety, since sleep and mood problems often travel together.

If any of these apply, the stage breakdown from a wearable is at most a supporting detail to bring to the appointment. The position statement from the American Academy of Sleep Medicine is explicit that such devices are not a substitute for medical evaluation.

What a wearable can and cannot tell you

Consumer sleep trackers are reasonably good at one thing: telling when you were probably asleep and when you were awake, and so estimating total sleep time. Their stage breakdowns are weaker, because the stage is inferred from signals that only loosely follow it.

The American Academy of Sleep Medicine published a position statement on consumer sleep technology in 2018 that makes two points worth keeping together. The devices are not cleared to diagnose sleep disorders and are not a substitute for medical evaluation, and the data they produce can nonetheless be useful in a conversation with a clinician as a record of patterns over time.

• Look at trends over weeks rather than a single night, since night-to-night stage percentages vary a great deal.
• Treat the stage split as a rough indicator and total sleep time and regularity as the more dependable parts of a report.
• A low "deep sleep" figure is not a diagnosis, and a high "sleep score" does not certify that sleep is healthy.
• Loud snoring, witnessed pauses in breathing or severe daytime sleepiness need a medical assessment, not a better tracker.

The risk of chasing the numbers

Researchers writing in the Journal of Clinical Sleep Medicine in 2017 described a pattern they called orthosomnia: people whose sleep worsened while they pursued the numbers their tracker reported, becoming anxious about scores and spending more time in bed to improve them. The case series was small and descriptive, but the observation is widely recognized by sleep clinicians.

If a tracker is making you anxious about sleep, the practical response is to stop checking it, or to look at it weekly rather than every morning. How you feel and function during the day is a better guide than any stage percentage, and it is the question a clinician will ask.