I used to have it happen a lot. The sympathetic nervous system gets into overdrive when we are anxious. I struggled with the nocturnal panic issues
when I was just getting a handle on the daytime panic! I think that we learn to suppress it, only to have it come out when our defenses are down.:( The more we practice our coping techniques...and lower our overall stressors however, the less it will be a problem...day or night.:)
Do you have a normal HR during the day?
What have you done in terms of anxiety management besides the meds? Do you have coping skills that help?
Maybe getting a little more of your story/history will help us to better advise you.:)
Talk to you soon,
S.C.