Why does the heart rate monitor randomly stop ?

I get the same problem, on occasion I will wake in the morning and the watch has stopped monitoring HR despite wearing the watch for sleep tracking.

Thank you for your answer. This happens independently of the charge level. If this is due to battery saving aspect, it would be logical to have a choice in the set-up to force always-on.

What do you mean by the HR stops?  Are you looking at it on your watch face?  In Garmin Connect?

All of the recent Garmin watches have 2 power modes for HR, a higher power mode while recording an activity, and another while not recording an activity.  Accuracy definitely goes down in the low power mode, but I still see it getting HR.  I don't wear my watch at night though, so wouldn't see it then.

I've now being wearing the S62 for the past 36 hours non stop and haven't seen these issues.  This morning, when I looked at GCM, it only showed HR data through 2AM.  But after syncing, the data was fully there until just now and it properly had my sleep data.  

I'm still a little confused exactly what you are/aren't seeing.

When I go to bed at night, the watch face is showing my heart rate.  I wear the watch whilst I sleep and when I wake up in the morning the heart rate is no longer showing on the watch face.  I click on settings, sensors and accessories and heart rate wrist HR is selected off.  I select it back on again and the heart rate shows on the watch face again.

In the month I have owned the watch, this has happened twice whilst I have been sleeping and once during the day.

This is the exact symptom. And you have to switch manually back on the HR toggle.Last week it was nearly once a day (first week of ownership)

Has the pattern of your OHR sensor being disabled had any pattern? For example, when you notice it is not on, and you go into the settings of your watch and find it off, is it possible you synced it to the Connect Mobile APP just prior? A good example, each morning when your sleep and other wellness stats are synced.

interesting question, i didn't note this, but it's possible. As a new toy, i frequently synced the watch and the app. Since a few days (an app update ?) it doesn't seems to happen for the moment.