Why am I waking up to a pounding heartbeat after I drink a lot?

Many things. If you mean drinking a lot of alcohol, the most likely reasons are dehydration, low blood sugar, and/or that alcohol, for all its pleasant side effects, is still a kind of poison. Asian & first nations heritage also holds a genetic defect that makes alcohol even more toxic.It also does not play will with other medications, especially for depression or anxiety. Ask your doc. Don't drink until then.