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.
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.
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.
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.