Pooh looked up at his clock, which had stopped at five minutes to eleven some weeks ago.

"Nearly eleven o'clock," said Pooh happily. "You're just in time for a little smackerel of something," and he put his head into the cupboard. -- From "The House at Pooh Corner," by A.A. Milne

For many of us, it is always time for a little something. Even after the gluttony of the holidays and the vows never to eat again, the hunger returns.

Humans get hungry for an important reason. Without that gnawing feeling that drives us to find food, the species would not survive. That is why there are a number of backup systems in the brain that prod us to eat.

"You can damage large parts of the brain and an animal can still know it must eat and regulate its body weight," says psychiatrist Mickey Stunkard, a specialist in obesity and eating disorders at the University of Pennsylvania. "Nature didn't want to leave it up to chance."

This insistent hunger mechanism is adapted to animals whose lives bounce between feast and famine. When food is scarce, evolution favors those who have been driven to devour and store up as much fuel as possible and then use the fat slowly during hard times.

But when food is abundant, as it is for most Americans, this mechanism needs some fine tuning. Some people lose touch with feelings of genuine hunger and continue eating even when they have had enough to satisfy their physical needs. The constant feasting leads to a weight problem, and that can increase the risk of high blood pressure and heart disease and reduce the chance of successfully losing weight.

One way out of this damaging cycle is to pay attention to when hunger strikes. If it has been several hours since a meal, then the sensation is probably a genuine reflection of the body's -- and especially the brain's -- impending fuel needs.

Several conditions combine to trip the hunger mechanism, according to Harry Kissileff, assistant professor of clinical psychology at Columbia University College of Physicians and Surgeons. The blood sugar levels drop once the last meal has been digested. The liver switches from making glycogen out of glucose to making glucose out of glycogen -- meaning it starts tapping stored energy. And adrenaline increases to release free fatty acids. All of these changes are registered in a tiny section of the brain called the hypothalamus, which is thought to regulate not only hunger and thirst but also temperature, aggression and sex drive. Then the brain releases a variety of chemicals that ultimately create the feeling of hunger.

Emotions, too, play a role in appetite regulation -- but an unpredictable one. "Stress and depression can make people overeat or undereat," says Jacqueline Crawley, chief of behavioral neuropharmacology at the National Institute of Mental Health. "We don't know exactly why."

One theory, she says, is that the neurotransmitters released by the brain during stress -- such as norepinephrine (which usually depresses appetite) or enkephalins and endorphins (which often stimulate appetite) -- can also, in some individuals, have the opposite effect.

Some hunger alarms are false. The stomach rumblings commonly considered signs of hunger have nothing to do with hunger. They are routine stomach contractions that occur every 90 minutes.

Luscious smells can trigger false hunger alarms, too. Aromas -- and even thoughts -- of good food can trigger saliva, digestive acids and more rumblings. But none of these necessarily means that your body needs fuel.

Hunger comes in many different forms, says Judith Wurtman, a research scientist in biochemestry at the Massachusetts Institute of Technology and author of "The Carbohydrate Craver's Diet" (Ballantine Books, $2.95).

In addition to the general hunger for calories, she says, "there is protein hunger and carbohydrate hunger." Though most people want a combination of proteins and carbohydrates, some people tend to overeat only in one of those categories. And the way to stop overeating is to give yourself the kind of food you crave.

"Don't fight it," Wurtman says. "It's a biological need."

Carbohydrate craving "stems from a very specific metabolic need for sweets or starches," she says, "which increases the amount of serotonin in the brain, which in turn shuts off the carbohydrate hunger."

Eating a protein, similarly, may shut off the protein hunger. But if it is really a sweet you want, a steak will not quell your "hunger." Only a pure carbohydrate, Wurtman says, will shut off the specialized carbohydrate craving.

Giving in to the craving needn't mean giving up on a restrained diet. It doesn't take much to satisfy that urge -- a low-calorie snack of popcorn or a couple of crackers or a cookie should suffice. And the snack should occur, Wurtman says, when the hunger strikes. Most people get cravings between meals -- from 2 to 4 p.m. or 9 to 11 p.m.

"The carbohydrate craving is not associated with the need for food," she says. "Premenstrual women have also been shown to have an increased hunger for sweets."

But don't expect instant results. It takes about half an hour to shut off the carbohydrate hunger. There is a drug available in Europe that can increase serotonin levels more quickly. But D-fenfluramine (marketed as Isomeride) won't be sold in this country for at least a year, Wurtman says. Taming the Cravings

Hunger experts offer this advice for taming food cravings:

If you crave a starch or a sweet, munch a carefully apportioned (under 200 calories) snack. Suggestions: 18 small pretzels, one bagel (nix the cream cheese) or a couple of oatmeal cookies. The snack should cause serotonin to be released in the brain, calming the hunger urge.

Protein cravers can eat a protein appetizer -- anything from a ricotta cheese puff to peanut butter on a stalk of celery -- then wait a half-hour before beginning the main meal. This gives the brain time to get the "full" message.

Consume hot liquids such as soup to calm the appetite. The heat and the high salt content probably keep the foods in the stomach longer.

Fill up on bulky foods. When the stomach is stretched it registers a full feeling.

Eat slowly -- that gives the brain the 20 to 30 minutes it takes to register you're full.

Try a diversionary tactic until the hunger wanes, since hunger tends to come and go in waves. A classic behavior modification technique breaks the habit of eating right away by doing something else -- take a shower, do the laundry. Nine times out of 10 the hunger will pass. But if you're still hungry, then its likely the body needs fuel and should be fed -- in moderation.lance writer.