Fenix 7 Solar vibrates at night for no reason and despite DND

Maybe a low heartrate alarm?

And then vibrates to stress user and increase heartrate?

Interesting idea, was really hoping it could be the solution. I then checked the settings. However, the heart rate alarm is disabled. Last night my Fenix vibrated 3 times again. The nightly vibration does not show anything on the display that triggers the alarm.
What could it possibly be.
That only happens at night, not during the day.

Check again for any alarm, timer or anything that can trigger an alarm. Also, disable alerts or any related settings. What I would do is turning everything off and then turn one thing on at one time. If that is not working, this is less convinient, however I would think to do a reset of the settings or a factory reset. 

I found it now. It was the alarm when disconnected from the smartphone. My smartphone is in the other room at night to charge. From time to time the Bluetooth connection disconnects at night. I have now deactivated this alarm. Many thanks for your help.

Where do you deactivate this alarm?

It is happening to me also.