Why does my garmin instinct show that I am very stressed when I am asleep?

Do you have an irregular heart beat? Heart Rate Variabilty is one of the parameters they use to measure stress.

No my heart beat is stable. 

There can be several reasons. Nicotin, alcohol, vaccination, late training, sickness etc.

Do you have stress every night? What does it look like? Post screenshots. If you got nights without stress, then compare what you do differently. 

I don't smoke, I don't exercise late & I only drink 1 night a week. I'm 55 and really only have the garmin to monitor my steps and cycling to work and back, also what my heart rate is, average low of 52. I just noticed on the stress monitor that I seem to be stressed when I'm asleep. 

What does it look like and has it changed recently (check the history)?

The biggest stress period during sleep is normally the first 2 or 3 hours, between 50 - 75, then I get some broken rest periods with some stress for the rest of the sleep period. My deep sleep averages 1 and half hours.