A sleepless night here and there, a stressful day at the office, or one too many Krispy Kremes can take their toll. But when you're constantly feeling drained, it might be time to look at what's bringing you down. One key to cutting back on sugar is having the right food with you so you don't head to the nearest vending machine. "The best intentions go out the window when you're not prepared," says Florida nutritionist Pamela Smith, RD, author of The Energy Edge. Exercise (Too Little or Too Much) When it comes to fitness, there are two ways to zap energy, Silber says. Overtraining depletes our energy reserves, breaks down muscle, and eventually makes us weaker, not stronger. Overdoing the workouts also suppresses the immune system, which in turn reduces our resistance to bacterial and viral invasion, Silber says. Dehydration "Most people don't drink enough water," says Scottsdale, Ariz., nutritionist Susan Ayersman of Kronos Optimal Health Center.