Sleep score: what rings and watches actually measure
Partilhar
A sleep ring or watch does not measure sleep. It measures movement, heart rate, heart rate variability and skin temperature. An algorithm then turns that into a number between 0 and 100. The difference between measuring and estimating explains almost everything that confuses you in the morning.
What your ring actually measures
The reference test for assessing sleep is called polysomnography. It reads brain waves, eye movement and muscle tone, in a laboratory, with attached electrodes. That is how light, deep and REM sleep are told apart.
Your ring does none of this. It performs actigraphy, using three sensors:
- accelerometer — how much you move and when you stop
- photoplethysmography — heartbeat and variability
- skin temperature sensor
None of these signals sees the brain. What the app calls deep sleep is an inference, not a reading.
Where the score gets it right
There are three things wearables do reasonably well. They are worth using.
- The time you fell asleep and woke up. Here the error is typically a matter of minutes.
- The trend across weeks. If your total sleep time dropped by 40 minutes a night over three weeks, that is real — even if the absolute number is wrong.
- Resting heart rate and temperature. They rise before you fall ill, after alcohol and on warm nights. That is useful information.
Where the score fails
- Sleep stages. This is the most advertised part and the least reliable. Validation studies show poor agreement with polysomnography when classifying deep and REM sleep.
- Nights spent lying down but awake. People with insomnia tend to stay still in bed. The sensor reads stillness and writes sleep.
- Comparing brands. Every manufacturer has its own algorithm and its own scale. An 82 on one brand is not an 82 on another.
Orthosomnia: when measuring makes sleep worse
There has been a name for this in the sleep literature since 2017: orthosomnia. It is the anxiety that comes from chasing the number the app considers good.
The pattern I see most often is this: someone sleeps reasonably well, opens the app, reads 68, and spends the day tired because the number said they should be. Expectation does the rest.
If you notice that your mood in the morning depends on the score, turn the notification off for two weeks and judge by your body. If you wake up rested, you slept well. That piece of data needs no battery.
What to do with the number
The rules I usually give:
- Look at the 7-day average, never at last night.
- Test one variable at a time. Last drink at 19:00 for 10 nights. Then the bedroom at 18 °C for 10 nights. One thing at a time, or you will not know what worked.
- Ignore the deep sleep percentage. It is the least solid figure on the dashboard.
- If the app says you slept badly and you feel fine, trust yourself.
If you want the basics before the metrics, start with the 12 sleep hygiene habits. And if you have been stacking up gadgets, first read what is worth it in sleepmaxxing and where your chronotype explains your schedule better than any score.
What no sensor tells you
The ring records that you moved 34 times during the night. It does not say why.
In most of the cases that reach me, the reason is mechanical. The pillow is too high for a back sleeper. Or the mattress no longer supports the hip and the body spends the night looking for a position. That is not fixed with data — it is fixed with the right measurements.
If your mattress is more than 8 years old, that is where I would start, before buying another sensor. Our mattresses are made in Portugal and come with a 10-year warranty. The Takumi (匠, master craftsman) is 26 cm high with medium firmness — it is the one I suggest to anyone who alternates between side and back sleeping in the same night. If you do not know where to begin, there is the complete guide to choosing a mattress, and we can talk for 10 minutes on the phone.
Notice
This article is informational and does not replace a medical assessment. A low score diagnoses nothing. But if you snore loudly, wake up with headaches or feel drowsy while driving, speak to your doctor: those signs may indicate sleep apnoea, and no ring detects it.
Ana Cristina, Sleep Advisor
