Does it happen to you?
@Angel9090
Well, it has happened to me on rare occassions
You can calm yourself down by trying to meditate if that helps..
I hope you're doing okay <3
@Angel9090
It could be a number of different health issues. If you are worried about it, I advise you to consult with a professional about it just to be on the safe side. 
If you use an alarm clock, the sudden awakening could cause a rapid heartbeat. It depends on how long it lasts, and how you feel besides the rapid heartbeat, for example: sweaty palms, dizziness, etc.
Again, I'm not a doctor and if this persists and causes problems, do check-in with a doctor. All the best!
@Angel9090 Yes, sometimes I have panic attacks like that where I wake up with my heart racing. i believe its my subconcious getting triggered by something, either in my dreams, or sounds around me.
@Angel9090
YES!!! when it happened for the first time I thought I was having a heart attack. When I went to the doctors my heart looked and sounded fine. Anxiety will I repeat WILL cause heart palpitations, rapid heart beat ect. Anxiety is the culprit. Here's somethings that help me. If you wake up with your heart pounding, remember your heart is healthy and can handle the speed it's going. Two try to control your breathing and remember it's going to be fine. I hope this helps. Please remember to relax you will be ok. Good luck!