Fortunately, by far the common course of holiday heart syndrome is benign. The abnormal heart rates will slowly resolve as the levels and effects of alcohol and/or other substances decline. That said, the risk is such that you don’t just want to sleep off an occurrence.
The symptoms you’d have are pretty obvious, given that you’d have been eating and drinking to excess. You’d also note that your heart was racing and perhaps pounding, as if it was attempting to jump out of your chest. Heart rates in the 120s are pretty typical for holiday heart syndrome (a normal heart rate is between 70-100 beats per minute). This tidbit is important to know; if your heart rate is higher than this, something more serious could be occurring. Under either circumstance, you need to be evaluated and treated in the emergency room setting with hydration and observation of the heart rhythm and rate.