Elevated heart rate detection drains battery overnight

It seems your friendly neighborhood ghost has been wearing your watch while you sleep. At least I can’t think of a more plausible explanation for records covering periods while the watch was not on your wrist. The graphs in the Health app should have a gap in records (blank period, missing bars) for when your watch had no human to monitor — or at least that’s how it shows on mine even just for the 30 minutes or so each evening when my watch gets charged, and I can certainly pick out a 3 hour gap where I fell asleep while the watch charged.

i think the best idea is to unpair the watch and re-pair before calling on the Apple’s paid support techs. Re-pairing is one of the first things they will consider because this resets and reinstalls the app software in tour watch. You may as well be one step ahead when they suggest it. Then if the problem is not cleared, see Contact Apple Support and pick your preferred initial contact method. With a little luck they will run remote diagnostics and might be able to see what is going wrong without a Genius Bar visit.

Check that your watch band is fitting snugly as Apple recommends. If it is slightly too loose the watch can move more than normal on your wrist, which can cause “noise” on the signal and false readings. This could prompt the watch to make extra measurements with extra load on the battery. To give you an example, while sitting quietly I can move my watch by hand and see the measured heart rate jump from around 60 to more than 120 - but the real rate never changed. Clearly this is a test example but it shows how an incorrect fit could mess with the results.

Also go into your health records and look at the recorded data for the period when you had heavy battery drain, and comparable resting periods when alerting was not enabled. They should be generally similar. You said it was while you were asleep, so your real resting heart rate is likely to be fairly low and steady, without big spikes. If it is erratic or you have many spikes, either you are getting interfering sensor noise or the spikes are real and you have something abnormal happening. If you think the spikes may be real you should consider consulting your doctor.

Why don't you put Watch on charge while you sleep?

You cannot over charge Watch with an original Apple charger.

Good point. That would make any records from the overnight period even more interesting as a malfunction marker, and also makes it easy to see the green LEDs if they are active at an inappropriate moment