Greteman's lunch choices commonly include fast food, a microwaved meal, or canned soup, all carbohydrate-laden foods. But high-carb meals immediately increase blood sugar levels, triggering your body to release insulin. Higher levels of insulin, in turn, decrease blood levels of the body's amino acids (the building blocks of protein). However, one amino acid remains on the loose: tryptophan, a precursor to serotonin. Insulin's effects allow more tryptophan to reach the brain, resulting in more brain serotonin—and increased fatigue and sleepiness.

To stay sharp after lunchtime, students with afternoon class loads need midday meals that include proteins (check the salad bar for beans to top your lettuce) and good fats, especially the omega-3s in oily fish (salmon, tuna, trout, sardines, and halibut), walnuts, and salad dressing containing canola or flaxseed oil. "There is some evidence that short-term loading with omega-3s does improve brain activity associated with learning, though the evidence regarding long-term effects is stronger," says Michael A. Schmidt, PhD, author of Brain-Building Nutrition (North Atlantic Books, 2001). A smart move for students: Keep a stash of canned tuna or salmon to tuck into quick, healthy sandwiches, and take along a baggie of walnuts and dried fruit to munch between classes. Don't be tempted by starchy lunch foods such as pasta, potatoes, and sweets, which may increase brain-dulling serotonin levels.