If you wake up with a hangover tomorrow morning on New Year's Day, medical science will have little comfort to offer. But short of staying home tonight, there are ways you may be able to avoid one.

"It's a very complicated process, the hangover," said Dr. Albert A. Pawlowski, chief of the alcohol research centers of the National Institute on Alcohol Abuse and Alcoholism. The major symptoms -- headache, shakiness, fatigue -- are really a form of drug withdrawal. Tension, disturbed sleep, heaving smoking and a drop in blood sugar -- all factors that can accompany drinking -- also contribute to the suffering.

Other than moderation -- the only sure preventive -- here is Pawlowski's advice:

Choose your poison. Part of the headache is due not to alcohol, but to impurities called congeners which impart flavor. Distilled liquors -- whiskeys, vodka, gin -- generally have fewer impurities than wine, which also contains solid material, pigments and tannic acid. But distilled liquors have a higher alcohol content, a disadvantage because they get you drunker faster.

Of distilled liquors, vodka and gin contain the fewest impurities, and among whiskeys, bourbon contains the most and Scotch the least.

Dilute your drink. It's best to slow your intake of alcohol, because all the pleasure of drinking occurs on the way up, Pawlowski said. Alcohol content in the blood peaks slowly, then falls off. But drinkers don't enjoy the downhill slide, even though they may still be drunk -- so once a partygoer reaches a comfortable high, he or she tends to keep drinking to maintain it.

With water is better," Pawlowski said, since mixing carbonated soda or tonic with alcohol will speed its absorption.

Eat. Having food in your stomach will slow alcohol's absorption and may modify its effect. Alcohol shuts off the liver's release of glucose, thus lowering blood sugar. Having a snack or a drink containing juice helps counteract this. Sudden drops in blood sugar may be one reason drinkers pass out -- although the effect of intoxication on the brain is another.

Keep other stresses to a minimum. "It's a real stress to the body to drink alcohol," said Pawlowski, because cells are bathed in a toxic substance that must be eliminated. Alcohol does violence to liver cells and disrupts the membranes of nerve cells in the brain. It also causes dehydration by inhibiting a fluid-conserving hormone and alters salt balance.

With all this to handle, the body doesn't need the additional challenges of heavy smoking, sleep deprivation and tension.

Scientists cannot explain many phenomena of drunkenness because they still know so little about the brain. For instance, Pawlowski said, no one knows why people feel sober once the alcohol content in their blood is on a downswing, even though it may still be high enough to impair coordination. He said this false sense of sobriety may be why so many partygoers drive -- and have accidents.

Memory lapses are another enigma. Some people, while drunk, may act normally and yet, the next day, experience a memory "blackout" spanning several hours. "One theory is that the information is never recorded in the long-term memory bank," he said. "The neural connections are there so [a person] can talk and do things, but none of the stuff is processed for long-term retention." Some experts now believe such blackouts are an early symptom of alcoholism.

Even "sleeping it off" is difficult when drunk. For the first few hours, while the sleeper is still intoxicated, Pawlowski said alcohol depresses REM sleep, a form of sleep during which we dream and which normally occurs on and off throughout the night. Then, once intoxication wears off, REM sleep is "made up" -- accompanied by dreams and tossing and turning. This is thought to be the reason many people wake up exhausted and irritable after a night on the town.

What to do when the damage is done? Drinking water before bed may reduce the headache of a hangover by preventing dehydration, although it's likely to make your bladder awaken you during the night. Pawlowski believes aspirin helps -- but acetaminophen (such as Tylenol) may be preferable, since aspirin exacerbates alcohol's irritating effect on the stomach lining.

His prescription for New Year's Day: "Rest. Nap. Watch the football games. Maybe that's what they're there for."