I also have experienced this problem recently, unfortunately. I've always had a higher heart rate (usually 80-100, average around 90), I've seen several General Practitioners and Internists throughout the years. (I'm 30) I've worn a Holter monitor and had a resting EKG. Nothing has ever been found. I'm also on Thyroid medicine and recommend anyone on Synthroid, Levoxyl or any synthetic Thyroid supplement- make sure when you get your levels tested, supposed to be every 6 months minimum, to ask for the more in depth test, as the simpler one doesn't show all the Thyroid info. But I have Hashimoto's Hypothyroidism and I guess this is the most common. I have small nodules and my Thyroid increases and decreases in size. My Dr. thinks I should be on a higher dosage to prevent the size increase of the Thyroid - but this does possibly make my heart rate higher. I don't know if it's dangerous, the Dr. doesn't seem to think so.
Two days ago I had the scariest thing happen, after a night out with friends (I drank only one vodka/cranberry and one beer) I awoke around 2 am to feel like my heart was going to eject out of my chest. I put my hand on my chest and could feel the pressure from the heart pushing on my hand. It was very scary, so I had my first full-blown panic attack and was shivering and trying to control my breathing. Breathing in through my nose and out quickly through my mouth helped and also walking around slowly, anything but sitting still.
I saw my Dr. yesterday and he says that alcohol and spicy food (which I'd also had- 7 stuffed jalepenos) can stimulate the VAGUS NERVE, which helps control the heart rate.
So he concluded that the alcohol and spicy food did this and caused the fast and pounding heart rate. I'm 30 and from age 21 - 25, I went out to clubs and bars and drank, sometimes heavily (4-7) drinks about 2-3 times a week, minimum. This NEVER happened then. Now I've slowed down a lot and I can't even enjoy a beer? I don't know what happened. What's even more strange is this happened about a year ago, but less severe...and then I drank occasionally with no problems, until two days ago! It's strange, I don't understand, and I drank random stuff each time. The first time I drank rose wine, the second time vodka/beer. But I've had red wine / tequila / beer without problems?!
My Dr. says my heart sounds fine, and I know I aggravated it with my worrying. But it's worrisome when you wake from a seemingly relaxing sleep and your heart is pounding out of your chest. The racing heart I can deal with, the pounding, not so much.