Skipping your workout to save energy actually works against you. In a University of Georgia study, sedentary but otherwise healthy adults who began exercising lightly three days a week for as little as 20 minutes at a time reported feeling less fatigued and more energized after six weeks. Regular exercise boosts strength and endurance, helps make your cardiovascular system run more efficiently, and delivers oxygen and nutrients to your tissues. So next time you're tempted to crash on the couch, at least go for a brisk walk-you won't regret it.

You don't drink enough water

Being even slightly dehydrated-as little as 2% of normal fluid loss-takes a toll on energy levels, says Amy Goodson, RD, a dietitian for Texas Health Ben Hogan Sports Medicine. Dehydration causes a reduction in blood volume, explains Goodson, which makes the blood thicker. This requires your heart to pump less efficiently, reducing the speed at which oxygen and nutrients reach your muscles and organs. To calculate your normal fluid needs, take your weight in pounds, divide in half and drink that number of ounces of fluid a day, Goodson recommends.

You're not consuming enough iron

An iron deficiency can leave you feeling sluggish, irritable, weak, and unable to focus. "It makes you tired because less oxygen travels to the muscles and cells," says Goodson. Boost your iron intake to reduce your risk of anemia: load up on lean beef, kidney beans, tofu, eggs (including the yolk), dark green leafy vegetables, nuts, and peanut butter, and pair them with foods high in vitamin C (vitamin C improves iron absorption when eaten together), suggests Goodson. Note: an iron deficiency may be due to an underlying health problem, so if you're experiencing these symptoms of iron deficiency, you should visit your doc.

Striving to be perfect-which, let's face it, is impossible-makes you work much harder and longer than necessary, says Irene S. Levine, PhD, professor of psychiatry at the New York University School of Medicine.