It's 3 p.m. and you can barely keep your eyes open. Your limbs feel heavy and, in mid-sentence, a large yawn has stopped you cold -- for the third time. It's not as though you didn't get the requisite eight hours last night, so what's going on? It could just be one of those days, but if you're out of energy on a regular basis, you might want to take a look at your diet.

Pam Lynch, nutrition consultant and sports therapist, says there are a lot of ways in which what you eat -- or don't eat -- could affect your energy level.

"If clients come to me complaining of low energy, I would look at their individual diets," she says. The food you eat may not be the only thing affecting your lagging spirits, but it makes sense that what goes in affects what you are able to put out."

Every person is different, says Lynch. "If they're leaving out red meats, or they're a vegan or lacto-ovo, there may be an iron deficiency." All of these diet choices provide little iron, and Lynch says at that point, she would send a client for hemoglobin testing. If you do have an iron deficiency, Lynch says prescription iron supplements would be necessary to build your energy back up. And don't worry -- if you're a little low in iron, you don't have to turn away from your veggie lifestyle. Lynch says eating whole grains and lots of vitamin C can help boost your iron intake.

If iron isn't your energy-stealer, another culprit might be the latest diet craze.

"Carbs are the body's preferred source of energy," explains Lynch. "If you're on a diet that really restricts your carb intake, you might not have as much energy."

Extremely low-calorie diets could also zap your energy because your caloric intake would be so much less than what you're burning off.

So, follow the Atkins plan or the South Beach diet, if it's right for you, but if you are experiencing a lack of get-up-and-go, you might have to adjust the diet to your individual needs.

"Body uses food for energy," says Lynch. "It sounds obvious, but if you aren't getting enough calories, you won't have as much energy."

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Try following the Canada Food Guide, for an idea of how many servings of whole grains, fruit and vegetables you need to maintain your energy. And, Lynch warns, don't get lured by the quick fix.

Refined carbohydrates, like cookies and cake, might give you a "sugar high," but it won't have the lasting power of whole grains and other complex carbs. As for energy drinks, Lynch says, "Look at the ingredient list. What is in that drink to give you energy?" Often, the answer is sugar and caffeine, neither of which will provide more than a temporary boost.

"I call caffeine a drug," laughs Lynch. "Each person reacts to the drug differently." If you're depending on coffee or Red Bull to get you through the afternoon, why not try eating a heftier breakfast, like whole-grain toast with scrambled eggs and an orange? Nutrient-rich food take longer to digest, so you won't get the instant sugar high -- but you won't get the inevitable crash either.

Waiting too long between meals can result in a decrease in energy, says Lynch. We're all so busy it's hard to schedule regular meals. Even if you eat a good breakfast, you'll have burned the energy long before dinner rolls around. Be sure to have snacks on hand, preferably ones with staying power, such as multigrain crackers and cheese -- not a bag of Cheetos.

It doesn't take much. Try revamping your meals just slightly. Replace white bread with whole grain, fruit drinks with 100% fruit juice and add a little protein -- grilled chicken, cheese, sunflower seeds -- to your salad at lunchtime. Add the foods your body loves to burn, and your 3 p.m. slump may become a distant memory.

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