Allow me to help you in a way that I possibly can.
First, using a stop watch isn't going to give you any results. And to be honest, I think it is insane to measure your heartbeat with a stop watch.
@Matt Godin, at 17, you should be using
fitness trackers for kids with a heart rate monitor. In my opinion, these are the real watches that can really tell whether you have an abnormal heart beat or not.
Garmin Vivofit Junior is a good product to try.
The other option would be to consult your physician. Honestly, you can't lie around with this problem for far too long, especially if it is something you notice quite often.
Unless you are sleeping after an intense workout, which is perfectly fine, you might need to seek medical help.
The other option is to learn how to
stop palpitation using home remedies.