Why your watch's sleep score and how you feel don't always agree

You wake up feeling great and your watch says you slept badly. Or the other way round. It happens to everyone, and it doesn't mean your watch is broken or that you're imagining things.

What your watch can see

A watch doesn't measure sleep directly. It estimates it from how much you move and from your heart rate, and uses that to guess when you fell asleep, when you woke and which stage of sleep you were in. It's good at total time asleep across many nights. It's much less reliable at sleep stages on any single night.

What it can't

Your watch doesn't know how stressed you were going to bed, whether you were lying awake but still, or how refreshed you feel. It also doesn't know what "good sleep" means for you. A score is the maker's formula, applied to everyone.

Which should you trust?

Both, for different things:

How you feel is the thing you actually care about.
Your watch is good at spotting slow trends you might not notice, such as sleeping a little less every night for a fortnight.

Patterns beat scores

The most useful questions are about patterns. Are you sleeping less than usual, or later? Is it every night, or only after certain days? Many people find their shortest nights follow the same few things: training late in the evening, a drink with dinner, screens in bed, or a busy day that's hard to switch off from. Once you can see the pattern in your own data, you can do something about it.

One honest test

If you think something is costing you sleep, change just that one thing for a week and compare your sleep with the week before. It won't be a perfect experiment, but it's far more informative than a single night's score, and it's about you rather than an average person.

Tempest describes your own data; it doesn't diagnose or treat anything. If you're worried about your health, speak to your GP or call NHS 111. In an emergency, call 999.