I own an Apple watch series 2 and I've recorded my heart rate during sleep for a week now. Twice during this period my heart rate during sleep shot up to around 110 for 1.5 to 2 hours and that seems very suspicious to me. Since I don't have any sleep problems (that i know of) I was wondering whether this was an inaccuracy of the heart rate sensor. So I wanted to know whether there is some consistent way to trick the sensor into recording double the actual heart rate (e.g. by pressing the watch against the wrist or something).
If you are concerned, you need to see a doctor.
That being said, dreaming and nightmares during normal REM sleep increase your blood pressure and heart rate. REM sleep lasts, on average, for about 20% of your total sleep time.
To calibrate your watch, you could always purchase a relatively inexpensive heart rate monitor to see if both devices sense the increase, and how accurate they are in comparison.
I have not heard of cases where the Apple Watch consistently records the double actual hearts for hours on end. If I were you, I would use a different type of device to measure your heart rate to ensure that you are actually seeing bad readings and not some kind of health problem.
Apple has a support page with tips on how to get the most accurate heart rate measurements:
In my own experience the most important is ensuring a good fit on the wrist. If you can easily get the watch too "loose" when moving around in bed, consider using a different kind of strap.