First of all, what actually is a hangover?
According to Dr. Rohsenow, the most valid definition of a hangover is "the set of subjective, unpleasant symptoms that occur after most alcohol has left the body after a single episode of heavy drinking." Hangovers generally appear after reaching a blood alcohol level of .10% or higher, with symptoms peaking once that level falls back down to zero.
It's worth distinguishing between signs and symptoms, here: in short, signs are observed by the physician, whereas symptoms are experienced and reported by the patient. In tests comparing a night of heavy alcohol consumption with an equivalent amount of placebo, the only validated symptoms included "thirst, head pain, stomach distress, and feeling tired or dizzy." By comparison, no objective signs were validated (i.e. heart racing or facial flushing).