I think bradycardia (slow heart rate) while lying down could be a part of a person's dysautonomia. My lowest heart rate last holter monitor came at night- 58, though I also had a bunch of numbers that showed my heart rate got up to over 100 nearly every hour, night and day. Maybe when that happens the next time, you could try standing (for at least one minute) and then take your pulse and document both the lying down pulse and the standing up one and report it to your doctor and see if they have anything to say about it.
This happens to me everynight, but my doc does not seemed concerned - although it does concern me. We plan to talk about it more after she seems my heart monitor results in a few weeks... I'll update then..
It seems to be common among a subset of POTS patients. I would just be mindful of it if you get rxed something like a beta blocker that lowers heart rate; in that case you'd want to discuss with your doc how close to bedtime is ok to take it.
an overdue update-- my heart rate on the holter was seen going as low at 40bpm.... and as high as 177... so my medications have been switched up, but I still see my heart rate go as low as 45 on my own heart monitor.. needless to say it is just another lovely part of the dysautonomia