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I went to sleep wearing my Apple Watch one night and awoke to a few hundred irregular heartbeat notifications.

They weren’t legit. The watch just can’t get a good heart rate reading if it’s not situated perfectly on your wrist, and while sleeping it was pushed one way or another often enough to be useless.

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That's probably the case with all HR monitors, no? If it's not securely in place, any HR monitor will have the same problem. I bought a knock-off velcro wrist strap for my watch and it keeps it in place. I have mixed success with traditional watch straps as my wrist size always seems to be in-between notches.

All the optical wrist-based ones, probably.

I imagine if I wore my chest strap monitor I'd have better results as long as I put some kind of jelly on the electrodes to make up for the lack of sweat through the night.

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