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Your watch says you slept badly. You feel rested. Which one should you believe?

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Medicine & healthAnalytic#measurement#self-knowledge#sleep#wearable-technology
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Its coordinates: 6.13 Philosophy of Medicine

An everyday case for the morning.

You wake up feeling fine, better than fine. Then you check the sleep score on your watch or phone: poor. Too little deep sleep, too many restless spells. By lunchtime you feel tired.

Who was right?

  • The device. It measured something you cannot observe in yourself: movement, heart rate and breathing through the night. Our sense of how we slept is notoriously unreliable; people who say they didn't sleep a wink are often found in sleep laboratories to have slept for hours.
  • You. "Rested" is not a hidden state that a wrist sensor can detect better than the person who has it. The device measures proxies and runs them through a model; your sense of being rested is the thing itself.
  • Neither, quite. "Slept well" bundles several things together (how long, how deep, how you feel afterwards) that usually travel together and this morning came apart.

A further question: if reading "poor" made you tired by lunchtime, what exactly did the device measure, and what did it cause?

Answers from your own mornings are as welcome as theories.

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4 answers

  1. Elin Hedqvist

    Fellow

    I work nights and sleep in the day, and my watch has never understood it. It decides I was "restless" every afternoon the neighbours mow the lawn.

    But here's my practical answer: what I need to know before a shift is whether I'm safe to look after people, and that is a question about how alert I am, not how my wrist moved. So I trust the feeling, and I check it against small things, like whether I misread the rota.

  2. Hetal Shah

    Fellow

    In the pharmacy we now get customers asking for sleeping tablets because of a score, not because of a bad night. One man showed me a week of graphs and said he felt fine but "the numbers are terrible".

    I don't think the device is lying to him. It measures movement and heart rate and estimates the rest. The trouble is that a number feels like a verdict. My father, who ran the shop before me, used to say a thermometer tells you the temperature, not whether you are ill. The same goes for sleep.

  3. Priya Raghavan

    Fellow

    The lunchtime tiredness is the most interesting part. On predictive accounts of the brain, feeling tired is partly an inference that draws on what you believe about your state, not only a readout of the body. So a credible-looking score can change the feeling.

    There's evidence for this. At least one study told people, falsely, how much good sleep they'd had, and their performance on attention tests followed what they were told rather than how they'd actually slept. Sleep researchers even have a name for anxious tracking of scores: orthosomnia. So the device may have measured badly and caused correctly.

  4. Maximilian Ortner

    Fellow

    A clinical note: your watch is not measuring sleep. Sleep is defined by brain activity, which a sleep laboratory records with EEG. A wrist device infers sleep stages from movement and heart rate, and consumer devices disagree with the EEG often enough that I tell patients to treat the score as a rough diary.

    But I would not crown "you" the winner either. There is a well-described condition, often called paradoxical insomnia or sleep-state misperception, in which people sincerely report hardly sleeping while the EEG shows a near-normal night.

    So the third answer is right, and it can be sharpened. "Slept well" has a part the laboratory can measure and a felt part only you can report. The person who calls the morning fine is reporting the second, and the watch is guessing at the first.