What Your Sleep Tracker Can and Cannot Tell You

Consumer sleep trackers infer sleep from movement, heart rate, heart rate variability, and sometimes temperature and oxygen saturation. They are estimates produced by proprietary algorithms, not measurements of brain activity.

Generally reliable

• Sleep timing and regularity. When you went to bed and got up, night after night — the most clinically useful thing most trackers produce.
• Total sleep time, approximately. Usually within a reasonable margin for healthy sleepers.
• Trends over weeks. Direction of change is more trustworthy than any single night.

Weak or unreliable

• Sleep stages. Stage classification against polysomnography remains modest, particularly for deep sleep and REM in disturbed sleepers.
• Wake after sleep onset. Devices tend to overestimate sleep in people who lie still while awake — exactly the population with insomnia.
• Composite "scores." Undisclosed weightings that are not validated clinical endpoints.

The behavioral risk

Nightly score-checking can increase sleep-related anxiety and performance effort, which worsens insomnia. In clinical work we often shift patients from nightly review to weekly review of timing and regularity only. See Sleep + Digital Health for how organizations should interpret this data at scale.