Deep sleep

What it is

Sleep cycles through stages. Non-REM sleep is split into N1 (light, transitional sleep), N2 (the bulk of non-REM sleep) and N3, deep or slow-wave sleep, named after the slow brain waves seen on an EEG.

Why it matters for training decisions

Deep sleep is part of what makes sleep restorative, and athletes naturally want more of it. But there’s no simple target number that applies to everyone, and the amount varies from night to night. The bigger, better-supported levers for athletes are getting enough total sleep, keeping it reasonably regular and limiting disruption.

How wearables measure it, and the limits

Sleep stages are properly scored with polysomnography (PSG) in a sleep lab, using brain, eye and muscle signals. Wearables estimate them from movement, heart rate and sometimes breathing.

• Apple Watch reports Deep (N3), Core (N1 and N2 combined) and REM. In Apple’s own validation, the most common error overall was true deep sleep being labelled as Core.
• An independent 2021 study of seven consumer sleep trackers against PSG found they detected sleep well, but their sleep stage results were mixed, and they performed worse on nights with disrupted sleep.

So your deep sleep number is an estimate. It’s more trustworthy as a trend over weeks than as a verdict on one night.

How P247 uses it

Deep sleep, as recorded in Apple Health, is one of the inputs to P247’s daily recovery score, along with sleep duration and sleep continuity.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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