What causes your heart to race when you suddenly wake up?

Heart rate: When we awake our bodies typically release adrenaline to speed up our hearts to make sure blood gets to the brain after sleeping all night . This sensation should last for only a few minutes.

Hypnopompic Stimuli: It's not entirely clear what circumstances you're referring to, but there is something called hypnopompic state which is a very stimulating dream-like environment just before waking. If you're talking about normal waking, it's a combo of increased cortisol, overnight drop in blood volume from very mild volume loss (you don't drink h2o while asleep) and positional changes as you get up.