Accuracy of Apple Watch Heart Rate Sensor?
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.