After the countdown ends, the confetti falls and the champagne bottles are empty , all that's left is a hangover.
Science might help you avoid a hangover this holiday season.
Even if you do end up with a hangover, there are a few strategies that can help -- and lots that don't.
It's probably safest to avoid driving and similar tasks when hung-over.
Along with party hats, fireworks and champagne, holiday revel often brings hangovers that can make the celebration-filled season seem less bright.
This year, science might help. Studies, both old and new, offer tips for what works to both prevent and treat hangovers -- and what doesn't. Along the way, the research can help prevent you from making a bad situation worse..
"As they always say, the best prevention is just not drinking too much in the first place," said psychologist Damaris Rohsenow, associate director of the Center for Alcohol and Addiction Studies at Brown University in Providence, R.I. "If you do drink too much, you might want to stay home and get over it for awhile before going to work."
There is no official medical diagnosis for a hangover, but you know one when you have one. Pounding headache, nausea, vomiting, stomach pain, sensitivity to light, trouble concentrating: The symptoms are impossible to ignore. Less clear is what causes them.
Dehydration is part of the story, said Janet Engle, a pharmacist at the University of Illinois at Chicago College of Pharmacy. Alcohol increases the body's production of urine, she explained, raising the chances that you'll lose more water than you take in. That, in turn, can cause headaches, dizziness and other symptoms.
To avoid dehydration, it can help to drink a glass of water between every alcoholic beverage. It can also help to choose more diluted drinks over shots and other highly concentrated options.
But there is more to the hangover story than just fluid-loss. Drinking too much alcohol can also cause blood vessels to dilate, the immune system to over-react, blood sugar levels to drop and the lining of your stomach to become irritated.
And hangovers only happen when a drinker's blood-alcohol level gets up to at least about 0.11 and then drops very close to zero -- usually about eight hours after the time of peak intoxication.
One theory among many for why that bottoming out induces a hangover involves the breakdown products of methanol, which leaves the body around the same time hangover symptoms start.
Alcohol also contains small concentrations of chemicals called congeners, which act as toxins in the body. Congeners get into alcohol through the fermentation of grapes or grains, as additives, or from aging in oak barrels. And congener levels explain why dark alcohols, such as bourbon and brandy, cause worse hangovers than light ones do, like vodka and gin.
Bourbon has 37 times the concentration of congeners compared to vodka. Scotch and whiskey lie somewhere in the middle.
Beer does not contain congeners, but its carbonation increases alcohol absorption in the body. And red wine contains tannins, which add symptoms of their own. As you decide what to drink, one of the worst things you can do, Engle said, is to pursue too much variety in the same night.
"The toxins in whiskey are different from the ones in bourbon or red wine," she said. "You don't want to have beer, then a shot of tequila, and then more beer. If you stick to one thing, you'll be better off."
Drinking just a little or not at all is, of course, the best prevention strategy. But if it's too late, you've had at least three drinks, and you feel drunk, Engle said, don't take over-the-counter painkillers to try to prevent the next day's hangover.
Combined with alcohol, acetaminophen can cause serious liver damage. Anti-inflammatories, such as aspirin, ibuprofen or naproxen, can harm a stomach that is already irritated by alcohol.
Only a handful of studies have actually looked systematically at whether supposed hangover remedies actually work the next day, Rohsenow and colleagues reported earlier this year in the journal Current Drug Abuse Reviews. But the results did offer some help.
Once the alcohol is out of your system, for example, there is data to support the use of over-the-counter medicines and a few prescription ones to help with physical discomforts.
One study found encouraging results for prickly pear cactus extract, which reduced the risk of having a severe hangover by 50 percent, especially after people drank dark alcohols. The extract also helped with nausea, low appetite, and dry mouth, though it didn't improve headaches, weakness or dizziness. Other work has shown that yeast and an herb called borage might help, too.
But there is no evidence to support most commercial products and herbal remedies that claim to cure hangovers. Simple sugars and B-vitamins also failed to prove their worth, as did artichoke and prickly pair.
Drinking more to stave off a hangover -- a common strategy also known as "hair of the dog" -- doesn't work, either. At best, the strategy simply puts off the moment when blood alcohol levels drop low enough for symptoms to emerge.
It can also help to be lucky. In one study, Rohsenow's team found that about 23 percent of people who drink enough to get hung-over don't get hangovers, probably thanks to their genes. Exactly how much alcohol it takes to get drunk also varies from person to person.
If you do wake up feeling bad, you might want to avoid heavy machinery.
In a study published this year in the journal Alcoholism: Clinical and Experimental Research, Rohsenow and colleagues found that people with hangovers did worst on tasks that required sustained vigilance and rapid decision-making. Their response times were 2 percent slower than those of people who had drank a placebo, probably because of reduced blood flow to the parts of the brain that are responsible for that kind of performance. And people who felt worst performed worst.
In other words, even after you've filled up on water and taken something to stop the pounding and the spinning, it's probably worth letting someone else drive on the morning after the night before.