Sleep stages and deep sleep

• Watches and rings combine a movement sensor with PPG heart rate; some add HRV, skin temperature and breathing rate.
• A sleep lab uses polysomnography (PSG), recording brain waves, eye movement and muscle activity.

• Chinoy 2021: seven trackers in 34 healthy young adults over three nights agreed inconsistently with the lab on stages.
• The 2026 ring meta-analysis (28 articles) says the devices are no substitute for detailed assessment of sleep architecture.
• Fitbit Sense 2 and Oura Ring were poor at identifying individual stages, especially deep sleep (Sleep Advances, 2026).

Source: Sleep, 2021 (Chinoy et al.), Journal of Translational Medicine, 2026, Sleep Advances, 2026

• The 2026 meta-analysis says finger-worn devices may suit long-term sleep versus wake tracking in healthy adults, or triage for severe sleep apnoea.
• It says they should not replace a sleep lab for diagnosing mild obstructive sleep apnoea.
• The NHS advises a GP visit for breathing that stops and starts in sleep, gasping, snorting or choking noises, or constant daytime tiredness.

Source: Journal of Translational Medicine, 2026, NHS

• Watch total sleep and how it shifts over weeks; the stage figures are the least reliable part.
• Frequent waking, or breathing pauses a partner notices, is worth mentioning to a GP.

How much sleep do adults need?

The NHS says adults need 7 to 9 hours.

Can a tracker measure deep sleep accurately?

Not precisely. Against a sleep lab, stage agreement was inconsistent and deep sleep was poorly identified. Total sleep and sleep versus wake are more reliable than stages.

Can my tracker tell me whether I have sleep apnoea?

No. A 2026 meta-analysis found finger-worn devices better at flagging severe sleep apnoea than mild, and said they should not replace a sleep lab for diagnosis.

• Insomnia, NHS
• Sleep apnoea, NHS
• Performance of seven consumer sleep-tracking devices compared with polysomnography, Sleep, 2021 (Chinoy et al.)
• Performance evaluation of finger-worn devices for sleep stage classification and sleep apnea detection - systematic review and meta-analysis, Journal of Translational Medicine, 2026
• Performance evaluation of consumer sleep-tracking wearables and nearables in healthy young and older adults, Sleep Advances, 2026