Of course, you know that a good night's sleep, regular exercise, and effective stress management can give you a much-needed boost. But to further figure out why you're slumping, you need to pinpoint the energy-sucks in your diet.
1. You go long stretches without eating. Every time you go more than two hours or so without eating, your blood sugar drops. Food supplies the body with glucose, which your cells use it to make the prime energy transporter, adenosine triphosphate (ATP). Every part of your body needs it. But when blood sugar drops, your cells don't have the raw materials to make ATP. And then? Everything starts to slow down. You get tired, hungry, irritable and unfocused. Grab a bite every two to four hours to keep blood sugar steady. 
2. Your breakfast is something like pancakes, white toast, and muffins. Instead, start your day with soluble fiber found in oatmeal, barley and nuts. It dissolves in the intestinal tract, protects against blood sugar spikes and crashes later in the day, and creates a filter that slows the absorption of sugars and fats. A smart start: cereals with at least 5 grams of fiber a serving and whole-grain breads with 2g per slice.
3. You're eating the wrong veggies. There's no such thing as a "wrong" vegetable, but you’d better pick cruciferous ones, like broccoli, cabbage, Brussels sprouts, cauliflower and kale. They contain isothiocyanates, compounds that activate a protein called Nrf2, which in turn generates mitochondria, the part of cells responsible for converting glucose into ATP. Toss broccoli into stir-fry; mix shredded cabbage with vinegar; or season cauliflower with turmeric, cloves cardamom, coriander and cinnamon.
4. You avoid iron-rich red meat. If you're iron-deficient, you could eat the best diet and still be exhausted. Women need about 18 mg daily until 51, and 8mg after that. Beef is the best source of heme iron, the form most easily used by the body; a 3-ounce serving has 3mg. You can get nonheme iron from plant sources, like kidney beans and spinach. To help your body absorb nonheme iron, eat vitamin C-rich foods (orange juice, berries, tomatoes) and avoid coffee and tea an hour after eating as tannic acids can block iron absorption.
5. You've cut one too many carbs. Our bodies run on carbs which are too bad they've gotten a bad rap. Carbs help your body burn fat without depleting muscle stores for energy. The ideal diet is 50-55% complex carbohydrates, 20-25% protein and 25% fat. Complex carbs provide energy as they're digested, while protein and fat, along with fiber, slow the digestion process so the boost lasts a good long time.