It's impossible to evaluate your sleep with only one number

iOS 26 brought a new score to the Health app: sleep quality. (It’s not exclusive to the Apple Watch; any band or watch compatible with iOS can contribute to that number.)

I’m skeptical of scores like this. It’s reductive and can be misleading to assign a single score to something as complex as sleep. And, in a great irony, my numbers (!) kind of prove that.

A few weeks ago I dug a band out of a drawer — a Huawei Band 8 — and started wearing it again. I’ve been more committed to working out and, especially for cardio, it’s useful to have heart rate visible in real time.

I chose this band partly because of the heart-rate sensor accuracy shown in tests by the channel The Quantified Scientist. It’s an honest, lightweight band that goes unnoticed, which led me to keep it on my wrist when I go to bed. Its long lasting battery helps as well.

Both Huawei’s app (which is… passable) and Apple Health indicate high-quality sleep. In nearly a month of recorded nights, Apple classifies my sleep as “Excellent,” with several days scoring above 95 (the scale runs from 0 to 100).

Huawei gave me an 82 for my September nights. Its app offers insights that point out what could be improved (more deep sleep, fewer awakenings during the night).

Someone looking at those numbers might think my sleep is wonderful. They’re misleading numbers: I have bruxism and very light sleep, and I need medication to reduce jaw tension while I’m out; otherwise I wake with my shoulders, neck and shoulder blades locked, sometimes painful and, even medicated, occasionally with a debilitating headache.

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One hypothesis for numbers so discrepant from reality is that these bands and watches monitor only superficial aspects of sleep. Insomnia, sleep stages, how many times someone wakes during the night. The Apple Watch, one of the most advanced of its kind, also flags apnea, although it doesn’t provide a diagnosis. Apple itself clarifies that the alert refers to “signs of sleep apnea” and, if it appears, recommends seeing a doctor.

Still, according to my hypothesis, less obvious aspects of sleep escape wrist-worn sensors. It takes other signals to detect bruxism and leg movements, for example.

The “gold standard” test is called polysomnography. I’m investigating the causes of my troubled sleep and mitigations for bruxism, and as part of that process I spent a night in a clinic hooked to a computer with wires and electrodes. You can’t compare a watch, however advanced, to this:

It’s kind of hard to sleep like this.

The results are detailed. Too detailed, to the point of being hard for a layperson to read. (The report’s conclusion does a good job of “translating” the data.) It’s a striking contrast with the reports generated by apps like Huawei’s and Apple’s.

The downside of polysomnography is that, as a rule, the data come from one uncomfortable night (because of the gear attached to my body) in a strange place. For me, I managed about 6 hours of sleep during a 7h30 time in bed, a little below my at-home averages — according to Huawei, 6h57 and 7h16, respectively.

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The push by tech companies into sleep (the same companies that have already colonized much of our waking time) catches my attention. There are startups focused on the topic, like Arianna Huffington’s Thrive Global and Eight Sleep, which sells “pods” (connected mattresses and bedding) and claims on its site that “members experience up to 25% more deep sleep.” Nothing like numbers to convince you to buy!

While drafting this column I remembered a book by American author Jonathan Crary, 24/7: Late Capitalism and the Ends of Sleep, published in 2013. For Crary,

Sleep is the only remaining barrier, the only persistent “natural condition” that capitalism cannot eliminate.

Is it still?

In my limited view, I don’t believe sleep difficulties are (only) an “externality” of capitalism. Other factors — genetic, for example — must share the blame. It would be naive, however, to dismiss Crary’s argument, who throughout the book cites the excess of screens (in 2013!) as one of the recent behavioral changes over the past decades that undermine sleep.

Sleep coincides with the metabolization of what we ingest during the day: drugs, alcohol, all the debris of interaction with bright screens; but also the flood of anxieties, fears, doubts, desires, fantasies of total failure or success. This is the monotony of sleep and insomnia, night after night. In its repetition and honesty, it is one of the irreducible remnants of everyday life.

Exams, consultations, smart watches and bands are not free. Perhaps the market has indeed sunk its teeth into sleep and, following a familiar playbook, is selling individual solutions to a systemic problem.