Vera Vandenbosch is a health-conscious person. She prefers salad for lunch and a modest portion of rice and beans for dinner. But come 4 p.m., her good intentions go down the hatch.

"By late afternoon I'll be so starving that I need sugar ASAP," Vandenbosch confessed. "So I'll have a black-and-white cookie or a Coca-Cola."

Vandenbosch is one of many Americans who, in spite of their knowledge of good nutrition, simply don't want to nibble on carrot sticks or even fruit in the hours before dinner. They want sugar.

The cravings are so real that snack companies have started incorporating them into their ad slogans.

"You know that tired feeling you get at 5 p.m.? Neither do we," boast the makers of Balance Bar, implying that eating their product will solve the problem (better than a Snickers, long advertised as a pre-dinner fix).

So why does this happen? According to dietician Meg Corcoran, it's the sum result of eating the wrong foods during the day.

"Most people do not drink sufficient water, and they eat processed and refined carbohydrates for breakfast and lunch," she said. White flours, bagels for breakfast, sweet sugary muffins, pancakes -- all of these things send your blood sugar way high, and then crashing down."

Hence the tired feeling. And the crash makes you crave the bad foods that gave you the rush to begin with, continuing the cycle, Corcoran said.

The solution? Corcoran recommends a high-fiber, high-protein diet that cuts way down on processed foods and starchy, white-colored carbs and includes a lot of water -- a diet that she says resembles the Jennifer Aniston/Brad Pitt-favored Zone diet.

"Have a high-fiber breakfast cereal with a little cottage cheese for protein. Lunch should include a fiber option such as sweet potatoes, brown rice, legumes, chick peas, lentil or minestrone soup and veggies," she says.

Do this, Corcoran said, and you'll find yourself preferring the snack she recommends: fruit and water with a little protein such as cheese or peanut butter (she prefers real food to nutrition bars).

But psychologists say there's more to cravings than just diet. People often consider a sugary or otherwise-unhealthy snack a "reward" for a tough day at the office.

"We're so conditioned to have food around all the time as part of any positive experience that bad foods make the experience even more enjoyable," said Oregon therapist Marilyn Sorensen, author of Breaking the Chain of Low Self-Esteem.

Pennsylvania psychologist Salvatore Cullari said the snack reward is really just a bad habit.

"You have to break the pattern," he said. "Replace food with exercise, music, relaxation, a phone call to a loved one, a bath."

Still, Corcoran vows that if you eat the balanced meals she recommends during the day, you'll keep your energy up without caving in to cravings.

"Then if you really want junk food, have it after dinner -- so you don't set yourself up for mood swings throughout the day," she said.