What your sleep tracker cannot see

I've worn a sleep-tracking ring every night for eight months. Each morning, the app greets me with a colourful chart: minutes of deep sleep, minutes of light sleep, and a score out of a hundred. Checking it had become part of my routine, like checking the weather. Here's my honest review, and a warning.

First, the good bits. The ring is light, the battery lasts about a week, and I barely notice it at night. Over the months, it showed me patterns I hadn't seen before. I go to bed much later at weekends, and my sleep is clearly worse after a glass of wine. Seeing that on a chart did more to change my habits than any article I'd read.

Accuracy is another matter. In a sleep lab, scientists measure sleep by recording brain activity, along with breathing, eye movements and muscle activity. A ring on your finger can't do any of that. It makes an estimate from how much you move and how your heart rate changes. Research suggests these devices are fairly good at telling whether you're asleep or awake. They're much less reliable on the different stages of sleep. The detailed chart is a rough picture, not a medical measurement.

Now the warning. For a few weeks last winter, I got slightly obsessed. A low score would put me in a bad mood before I'd even got out of bed. I started going to bed earlier and lying there for ages, trying to push the number up, which only made my sleep worse. Sleep specialists have seen this in patients too. They call it orthosomnia, from Greek and Latin words meaning 'correct sleep'.

Would I buy it again? Probably, but I'd use it differently. These days I look at weekly trends, not nightly scores, and if I wake up feeling rested, I ignore a bad number. A friend's tracker flagged very irregular breathing at night, and she went to a doctor for a proper test, so these gadgets can help. If yours starts making you more anxious, though, take it off for a while. I took mine off for two weeks in January and slept better than I had all winter.