Venu 4 not recording heart rate and rebooting during sleep

Have you tried a hard reset? Hold down to top button until the watch turns off. How do you know it reboots while you sleep?

I've tried the hard reset but that doesn't change anything. I've also tried the 'Delete Data and Reset Setting' option, but that also didn't work.

Because I normally have my morning alarm on my watch, but the first day it happend I woke up naturally half an hour after the set alarm with the blue 'startup' triangle on my screen. 2 days later I had to go to the toilet at 5 am, and the blue triangle was on my watch for about 15 seconds and then disappearing for about 5 seconds before appearing again. That happend till around 7 am that day.