I have been experiencing these same symptoms (as soxfan) for about 13 years now. I believe what you are experiencing are either PAC's (Premature Atrial Contractions), or PVC's (Premature Ventricular Contractions). The parts of the heart (Atrium and Ventricle) beat out of their normal order, causing the sensation of a HARD beat (not fast).
I have noticed that the discussions in this thread are referring to two different conditions. Some people are talking about Tachycardia, POTS, heart racing, etc., which cause the heart rate to elevate to a scary level. Others, like Soxfan and Kisekishiawase, are talking about heart pounding (as in beating really intensely, but not fast), which is what I experience. This is where a person can feel the heart pulsating all the way down to the fingers, and you can actually see your chest thumping/bouncing. It feels like it's going to burst out of your chest at any moment. All the while, your heart rate is still generally at 70-80 bpm. I just wanted to clarify the difference.
One day back in about 2002, I was at the gym working out on the rowing machine, all of a sudden my heart started going berserk! I thought I was going to have a heart attack or something! I began having these sorts of episodes about once a month at first. After a few months, it was about once a week. Within a year, I was having them daily. I had gone to the doctor a few times to discuss it, and they did a stress test EKG on me. Found nothing too out of the ordinary, just an occasional PAC. Around the time I began having them daily, they put a Holter monitor on me. They discovered that 30% of ALL my heartbeats were PAC's!
Today, it is almost constant 24/7. It is less noticeable when I am standing or sitting, but still there. When I lay down it's much worse. And when I lay on my left side it's intolerable. Right side is a little better. Laying on my back seems best. In the morning, they are more gentle, over the course of the day, they get stronger. The closer to bedtime I eat, the worse they are.
It is extremely worrisome, and stressful. And like many of the posters here, I was having, and still have trouble sleeping. Over the years, I've been able to experiment, and identify some things that help me (or don't help).
I don't have high blood pressure, I am not overweight, and my heart rate is normal.
I have hypo thyroidism, and take Levothyroxine, 125 mcg. I wondered if that was contributing to my heart pounding, so a year ago, I stopped taking them for about 2-3 months. That didn't help the heart pounding though, and made me feel lethargic and sick, so I resumed taking the thyroid meds.
I've taken multi vitamins, and Vitamin C. They didn't help.
I've tried drinking tons of water. That helped with other issues, but not my heart.
I bought a RESPeRATE device, that sort of uses bio feedback to lower heart rate/BP. With that I learned that with deep breathing exercises I could diminish the heart pounding to a level that would allow me to relax and fall asleep. After awhile, I didn't need the device anymore to do it. The device didn't solve my problem, but it gave me a tool I could use to manage the cycle of stress/heart pounding/more stress/more heart pounding, etc.
I also did an exclusionary diet, where I cut out all caffeine, soda pop, alcohol, nicotine, junk food, sugar, meat, etc., then gradually reintroduced things back in. I found that by cutting all that out of my diet, the heart pounding almost completely stopped!
The biggest contributing factors ended up being sugar, nicotine, caffeine, and alcohol, in that order. The biggest problem is sugar. There is sugar added to almost everything we eat or drink. It's in places we don't expect, like ketchup, salad dressings, mayonnaise, most sauces (including spaghetti sauce), most fast food has sugar sneaking into burgers, burritos and tacos, bread, buns, and the list goes on. I try to buy things that have reduced or no sugar, which means reading a lot of nutritional information labels. I still allow myself to eat a piece of cake or pie or some ice cream once or twice a week, but I pay the price when I lay down to go to sleep (definitely will have some heart pounding going on).
Now I only drink decaf coffee, and I have quit smoking, rarely drink soda pop .
As far as alcohol, I have noticed that even though a couple glasses of wine helps me relax and reduces the heart pounding in the short term, after a couple hours the pounding is worse than it was before I drank the wine. I still drink, but try to limit it to weekends.
I've also learned, like one poster said, that if I exercise (in my case jogging) at least every two days, it also helps reduce the pounding sensation at night.
Overall, I've learned that I can have some control over the symptoms, and if I'm really diligent, I can almost eliminate the pounding.
Hopefully, these observations can be of help to some of the people who are suffering from this debilitating problem.