Praying to the porcelain god, your heart races and you wonder if someone has chopped an axe through you head. Why, why, why? you ask.
Well, you drank too much, silly.
Most people who consume alcohol have had a hangover at least once in their lives. And some of those people raising their glasses on New Year's Eve may be clutching their heads and bellies on Jan. 1.
"A hangover is a metabolic storm," said Dr. Joel Saper, founder and director of the Michigan Head-Pain and Neurological Institute in Ann Arbor, Mich. "It is a series of biological changes that occur in the body."
Key symptoms of a hangover include a (usually throbbing) headache, nausea, diarrhea, excessive fatigue and extreme thirst and dehydration.
So what's actually going on in the body once the hangover sets in? Doctors say they can't be entirely sure.
"Hangovers are not entirely understood, but dehydration, electrolyte and hormonal abnormalities, low blood sugar and direct toxic effects all contribute," said Dr. Andrew Yacht, vice chairman of the department of medicine at Maimonides Medical Center in Brooklyn, N.Y.
The liver breaks down alcohol into a byproduct called acetaldehyde, which is more toxic to the body than the alcohol itself, and is the reason for the post-drinking side effects.
But, as many people have noticed, every person reacts differently to alcohol. Some people have a few sips of alcohol and feel terrible the next day, while others can consume drink after drink into the wee hours of the morning and feel great.
That's because it's not just the amount of alcohol that goes into the body that influences the hangover.
Age, genetics, medications, diet, immune systems, weight and gender all can come into play.
"These are generalizations, but people who are heavier tend to be more prone to hangover headaches, and women are more prone to the effects of alcohol than men," said Saper. "If you have a weakened system to begin with, you're probably going to be more vulnerable to the effects of alcohol, as well."
Saper said that there are two different kinds of headaches that come with drinking: the "soon-after" headache and the "morning-after" headache.
Most people who have a soon-after headache have suffered from headaches and migraines in the past. The headaches are genetic, where the brain changes so that it overreacts to internal changes like hormones and external changes like a sip of alcohol.
Red wine and amber liquors have been found to cause more headaches because they contain large amounts of congeners, chemicals that form in the alcohol during the fermentation period.
A good rule of thumb is that the darker the drinks, the more congeners the drink contains (and potentially the worse the hangover symptoms).
But congener content shouldn't be the be-all end-all when deciding a drink of choice during a heavy night of drinking.
"If you drink a little bit of brandy and a lot of white wine, you're going to get a hangover from the white wine," said Yacht.
If you want to avoid a hangover, doctors say, don't drink. But, if the booze already is flowing, there are a few things drinkers can do to help make the morning after more tolerable.
For one, sip slowly.