It's three hours into the workday and you're already exhausted.

You skipped breakfast, but you're not that hungry for lunch, because you grabbed a doughnut at the office meeting. Maybe you'll get a salad later, but for now, a bag of chips and a diet cola seem the best way to get a quick energy boost.No wonder you feel lousy. What you eat, and when, makes a difference in how you feel all day.

"If you blame your lack of energy on your busy schedule or assume you were born this way, think again," says dietitian Elizabeth Somer in her book "Food & Mood" (Henry Holt, $25).

Breakfast, for instance, is just that. "It breaks your overnight fast and restocks your dwindling energy stores. Expecting your body to shift into full gear without stopping to refuel is like assuming your car will run on empty."

It's important to get a basic number of nutrients daily for a healthy diet. But if our needs ended there, we could all drink a vitamin-and-mineral-packed beverage in the morning and forget about eating the rest of the day.

The body is constantly processing the demands of our muscles, oxygen supply and blood sugar levels, which vary according to activity level. A large, fatty meal just before bedtime, or the pie and ice cream consumed as a midnight snack, have to be broken down by a body at rest. That body is burning calories much more slowly than that of someone who walks a mile or two after a feast.

The result? You may wake up feeling sluggish and stuffed and not inclined to eat a breakfast of fruit and grains to help you through the rest of the day. Many people also find it hard to concentrate at work after a heavy lunch, when energy is being diverted for digestion.

That doesn't mean the traditional "three squares" are the answer.

Eating at regularly scheduled periods throughout the day keeps health and energy levels high, according to the "Tufts University Guide to Total Nutrition" (HarperPerennial, $15). But "six small well-balanced meals can be as good as three hearty ones," says Stanley Gershoff, dean of the School of Nutrition.

The problem for many people is that several small meals easily turn into snacks - a handful of corn chips, a slice of pizza at happy hour. "Grazing" works well only if it's planned, so that you consume fruits, vegetables, grains and protein throughout the day, for a healthy balance.

Other tips:

- Try to eat the last meal of the day early. Take a short walk after dinner or keep moving by doing chores. Keep late dinners to soup and bread or a sandwich.

- If you are invited to a big dinner out, get your body back up to speed the next day by eating regular, light meals of fruit, vegetables and by drinking lots of fluids.

- Coffee and tea should be treated as beverages to complement a meal, not as liquid uppers. Caffeine can speed you up for a while, but it wears off, and it makes you more tired if you have sweetened your drink with sugar and cream.

- Still dragging? It may be thirst that's bringing you down. Drinking water helps beat fatigue because it's essential to the body in regulating temperature, maintaining blood pressure and carrying oxygen to your muscles. Consider reaching for glasses of cold water instead of a sugary pick-me-up.