What a sleep score measures, and what it guesses
Your wearable turns a night into a number out of 100. Some of the inputs are solid measurements and some are estimates.
Kivorent News desk, , 2 min read
What to know
- A sleep score combines how long you slept, how broken the sleep was, estimated sleep stages and overnight heart data.
- Total sleep time is the most reliable input. Sleep stages are the least.
- Scores from different brands use different formulas and cannot be compared.
Most watches, rings and bands greet you with a sleep score. It looks precise. The formula behind it is each company's own, and the ingredients vary a lot in how far you can trust them.
The ingredients
Nearly every sleep score draws on the same handful of inputs.
- Total sleep time. How long you were asleep.
- Sleep efficiency. The share of your time in bed that you spent asleep.
- Interruptions. How often and how long you were awake or restless.
- Sleep stages. Estimated time in light, deep and REM sleep.
- Overnight heart data. Resting heart rate, heart rate variability and sometimes breathing rate.
- Timing. Whether you went to bed and woke near your usual hours.
How the device knows you are asleep
Two sensors do most of the work. A motion sensor detects stillness. An optical heart sensor tracks your pulse. When you stop moving and your heart rate settles into a sleeping pattern, the device marks you as asleep.
This works well for the big picture. It struggles with lying awake and still: a wearable often counts quiet wakefulness as sleep, so it tends to overestimate sleep time for people who have trouble falling asleep.
The stage problem
In a sleep lab, stages are identified from brain activity recorded through electrodes on the scalp. A wearable has no access to brain activity. It infers stages from movement and heart patterns, using a model trained on lab data.
The result is an educated estimate. Wearables are reasonably good at telling sleep from wake and much less consistent at telling the stages apart. Two devices worn on the same night can report different amounts of deep sleep.
How much to trust each input
| Input | Reliability |
|---|---|
| Total sleep time | Good for most people |
| Bedtime and wake time | Good |
| Overnight resting heart rate | Good |
| Number of awakenings | Fair |
| Minutes of deep and REM sleep | Rough estimate |
Using the score well
- Watch the trend. A week of falling scores tells you something. Thursday's 71 against Wednesday's 76 does not.
- Look at the inputs. If the score dropped, find which ingredient moved. A late bedtime is fixable. A lower deep-sleep estimate may be noise.
- Do not chase the number. Worrying about a score can make sleep worse. If checking it makes you anxious, check less often.
- Stay with one device. Comparing your score with a friend's on another brand compares two different formulas.
When to look past the wearable
A sleep score cannot diagnose a sleep disorder. Loud snoring with pauses in breathing, regularly waking unrefreshed after a full night, or falling asleep during the day are reasons to talk to a clinician.
One of the inputs deserves its own explanation. See heart rate variability: what your watch is measuring.
This article is general information about how technology works. It is not medical advice. Talk to a clinician about your own health.
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