A wearable reports how much deep sleep you got. It did not measure deep sleep, and understanding what it did measure explains both its usefulness and its limits.
Sleep staging is defined by brain activity
Clinical sleep staging relies on recordings of brain electrical activity, eye movement and muscle tone, scored in short epochs by defined criteria.
Those criteria are what the words light, deep and REM refer to, and none of the required signals can be captured from a wrist or a finger.
A consumer device is therefore estimating a clinically defined quantity from inputs that do not include the thing the definition is based on.
The inputs are movement and cardiac signals
Most devices combine accelerometer data with heart rate and variations in the interval between beats, then apply an algorithm trained against laboratory recordings.
These signals do carry real information, since stillness, heart rate and its variability differ across sleep states in ways that are statistically useful.
What they cannot do is resolve individual transitions reliably, particularly between the lighter stages, which is where most of the disagreement with laboratory scoring occurs.
Total sleep time is the reliable output
Distinguishing sleep from wake is a much easier problem than classifying stages, and devices do it reasonably well for most people.
Bedtime, wake time and rough duration are therefore the numbers worth attending to, and they are also the numbers most people actually need.
Accuracy falls for people who lie still while awake, since stillness is the primary evidence the device has for sleep.
Proprietary scores are not comparable
Each manufacturer computes its own composite score from its own weighting, and the formulas are neither published nor standardized across brands.
A score of the same value on two devices describes two different calculations, so comparison between products or between people is meaningless.
Within a single device over time, the same formula applied consistently does allow relative comparison, which is the appropriate use.
When the number becomes the problem
Anxious attention to nightly scores can itself disturb sleep, a pattern clinicians have described in people who become preoccupied with tracking data.
If a report is producing worry about a night that felt fine, the report is worth less than the experience it is contradicting.
Persistent daytime sleepiness, loud snoring with pauses in breathing, or long-standing insomnia are matters for a doctor, and no consumer device can evaluate them.