What Is Deep Sleep and How Is It Estimated?
Deep sleep (slow-wave sleep) is the stage with slow brain waves that is common early in the night. Laboratories measure it with EEG; wearables estimate it from movement and heart rate, which is less accurate.
Key points
- What it is. The deepest non-REM stage, with slow brain waves.
- How it is measured. Polysomnography uses EEG, eye and muscle signals.
- What to do with the number. Treat it as a rough guide. Total sleep time and regular timing matter more.
What it is
The deepest non-REM stage, with slow brain waves.
How it is measured
Polysomnography uses EEG, eye and muscle signals. Wearables infer stages from motion and heart-rate patterns.
What to do with the number
Treat it as a rough guide. Total sleep time and regular timing matter more.
Frequently asked questions
Can I increase deep sleep?
Regular schedules, exercise and avoiding alcohol close to bedtime support healthy sleep, but guaranteed boosts do not exist.
Are deep-sleep percentages reliable?
Not on consumer devices.
Sources
- de Zambotti M, et al. Wearable sleep technology in clinical and research settings. Med Sci Sports Exerc 2019;51(7):1538–1557. PubMed
Links go to PubMed searches for the paper title or to the official guideline or regulator page. Standards and regulations are revised, so check the current edition before relying on them. Mattress and pillow research is limited; treat findings as a starting point.
Keep reading
Sleep Technology: Trackers, Apps and Smart Devices Explained
What sleep trackers, apps and smart devices can and cannot tell you, with the evidence on accuracy and who benefits.
Read more →Sleep technologyWhat Is a Sleep Tracker and How Does It Work?
How wearables and under-mattress sensors estimate sleep, what they get right and wrong, and who benefits.
Read more →Sleep technologyHow Accurate Are Consumer Sleep Trackers?
What validation studies find: good at total sleep, weaker at stages and wake detection.
Read more →This article is general information, not medical advice. If you have persistent pain or sleep problems, please speak to a doctor.
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