Why does my heart race and my blood pound when I wake up from a nap?

The same thing happens to me occasionally, and I've always attributed it to waking up with a start - either scared because I fell asleep without intending to, or surprised to find myself waking up during the middle of the day.

Semi-seconding the above-- could a http://en.wikipedia.org/wiki/Hypnic_jerk be causing you to wake up?

I don't think it's hypnic jerk because I have those at night and am occasionally woken up by them, but it's a much different sensation. Hypnic jerks are just little twitches, right? Whereas this is a very specific feeling that persists when I'm already awake.

I'm a terrible napper. The times I do manage to drift off for a nap, this will usually happen to me exactly as you describe. I usually get a general sense of panic with it as well, seemingly from nowhere.

A little panic. I used to get that too ... mostly when I knew I could only nap for a finite period and was worried about waking up in time ... and when I lived with the boring man I would get them too...Sort of a panic that said, "Oh no. This really is your life. It wasn't a dream." It helped me to have at least half an hour to nap and to set my alarm to something semi-soothing if possible (harder when you're using your cell phone as an alarm clock.)

I would get it checked out just in case your heart has something funny going on, or perhaps your thyroid. If it is anxiety related, sometimes the anxiety that these things cause makes them worse in a kind of feedback loop. Have you ever tried to record your pulse rate at these times?

I know you say you're not unhealthy, but do you snore, or is there any chance you have sleep apnea? Finally, do you nap in a position different from normal sleeping? I suffer from sleep apnea and before I started using a CPAP machine, I'd wake myself up gasping for air, heart pounding, after not having breathed for a few seconds. If I try to nap without the CPAP, I'll wake myself up from snoring.

kimota is right, apnea will do that to you.

Circadian rhythms affect various body systems, including temperature, blood pressure and pulse rate. When you sleep at night you're cooler, have lower blood pressure and a slower pulse rate. This isn't true when you sleep during the day, so you'll feel different when you wake up. Various hormones, such as cortisol, also follow the circadian rhythm, which could account for anxiety and panic upon waking from naps.