If walking to your mailbox or making it through the day without a nap seems beyond your current energy level, you may be suffering from fatigue. It has many causes, including stress, medications, overwork, sleep difficulties, or illness. If your fatigue is chronic or extreme, ask a doctor to check you out. But once you’re certain that your tiredness doesn’t stem from a medical problem, it may be time to reconsider your diet. Plenty of all-too-common eating habits sap energy—and plenty of relatively simple fixes can inject more oomph into your day.