I suspect not! No more than you'll get an accurate reading with a starlit sky on the mind.

I can't claim to be absolutely medically correct on this but two things to say:

1. I sometimes take a reading last thing at night using an upper arm BPM - if I am relaxed and all intend to do is snuggle a bit further under the duvet to go to sleep. This gives a good resting HR. Sometimes I do the check in the morning too soon after I am reasonably awake and have just shuffled above the duvet. HOWEVER - I am told that the most accurate reading in the morning is after you have woken and been to the loo to do functions in at least liquid kind, if not in the opther way too.

At night my resting HR can be down to 48. In the morning it's quite a bit higher in readiness for the day - mo0re like 60 something.

2. I had cause to accompany a friend to their cardiology appointment the other day, and the cardiologist said that they are generally unhappy with anyone's HR dropping below 56 bpm during sleep.

As it was not my appointment I didn't like chipping in and saying that mine is lower than that before I even go to sleep! I guess if you're exercising there has to be some give