Some nights you stay up very late and need to get up early, so naturally you feel tired. But sometimes, the root of your fatigue isn't so obvious, and everything from a hidden health issue to your gym habits could be to blame.Here are 7 reasons you could be dragging--and how to regain more energy than you ever remember having.

1. You're dehydrated
Healthy men and women who failed to replace a mere 1.5% of their water weight experienced mood swings and low energy levels. The study authors suspect neurons in your hypothalamus--the brain region responsible for controlling things like hydration and body temperature--send mood-altering messages to the rest of your brain as an early warning to drink more water.
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Your fix:[/b] Drink up, and ditch that 8-glasses-a-day guideline: A one-size-fits-all water measurement won't work since your hydration needs vary based on things like the weather and your workouts. In general, you should have to pee at least once every three hours and your urine should have a light lemonade-colored tint.

2. You're low on B12
Your body needs vitamin B12 to make red blood cells and keep neurons functioning properly. Deficiency decreases the amount of oxygen your blood can carry through your body, leaving you with that sleeping-with-your-eyes-open feeling. As you age, you produce less of a protein called intrinsic factor, which helps you process the nutrient.Because only animal foods naturally contain B12, vegetarians and vegans face an elevated risk of running low. Even low or borderline levels--not necessarily full-blown deficiency--can wear you down.

[b]Your fix:[/b] If your fatigue comes along with forgetfulness, restless legs, or numbness and tingling, consider B12 deficiency as a potential culprit. Ask your doctor or nutritionist for a blood test of your levels. If you're low, you may need supplements. Your doctor will tell you how much to take, but typical doses range from 100 to 500 mcg. Choose a formula labeled "methylcobalomin" instead of "cyanocobalamin." Note that supplements will only boost your energy if you're low to begin with; unlike caffeine, B vitamins won't give you an added oomph if you already have ample stores.

[b]3. You're overwhelmed with stress[/b]
Trying to do it all comes with a huge downside. Normally, your levels of the stress hormone cortisol run highest in the morning and dip down at night, helping you maintain a normal daily rhythm. But chronic stress throws this pattern out of whack in either direction. If your body remains on constant alert, your cortisol levels may never fall off at night, disrupting your sleep. Or, your adrenal glands may eventually fall behind in cortisol production, leaving you sleepwalking through your morning.

Your fix: You can't always control the sources of stress, but you can change your reaction. Mindfulness practices have been shown to ease stress and fatigue in people with chronic medical conditions like rheumatoid arthritis and multiple sclerosis, and they work for healthy people, too. Can't spare 15 minutes a day to meditate?

4. You have hidden heart disease
Studies show that half of women who had heart attacks said they had trouble sleeping and felt unusually fatigued in the weeks beforehand. Weariness and shortness of breath when you exercise, climb stairs, or otherwise exert yourself should also raise a red flag. Blocked arteries or a weak heart muscle reduce blood flow, preventing your muscles and tissues from getting the oxygen they need to function properly.

Your fix: Get to the doctor, especially if you've suddenly lost your get-up-and-go or if you have other strange symptoms, such as chest pain, anxiety, or trouble concentrating. He or she may recommend a stress test or an echocardiogram to screen for heart disease.

5. Your iron levels are too low--or too high
Most women know anemia leads to fatigue. But don't assume popping iron supplements will pep you up. Yes, low iron levels lead to poorly formedred blood cells that deprive your body of refreshing oxygen. However, getting too much iron can wear you down as well. Your body uses vitamins, minerals, and energy to rid your system of the excess, leaving you with little left to run on.

Your fix: Consider your risk factors: Iron deficiency often strikes vegetarians and vegans, people with digestive diseases or thyroid problems, women on hormonal birth control, and those with a very heavy menstrual flow. On the flip side, high levels can run in families or result from taking supplements, and often cause other signs that include feeling cold, thinning hair and nails, or dizziness when you stand up. Striking the right balance is important, so don't take iron pills on your own. Talk with your doctor about yearly blood tests to check your levels. If they're abnormal, have them checked monthly until they level off, then every three to six months until you steadily see normal readings.

[b]6. You're not working out[/b]
Especially when paired with chronic stress, too much time spent sedentary drains your fuel tank even though you're merely idling. Picture it: A stressful day at work cranks up your cortisol andblood glucose levels, triggering your knee jerk reaction to fight or flee. But when you spend your afternoon and evening barely moving between your computer screen and your couch, you never release that energy and tension. This can keep your engine revved and disrupt your sleep at night--or burn out your body's cortisol factory so much that you're dragging the next morning.

Your fix: Start moving if you're sedentary. Women who get the government-recommended 150 minutes of moderate exercise or 75 minutes of vigorous activity per week report less fatigueand more energy and vigor than those who don't. If you're using exercise to help you sleep better, you'll need to move consistently for a few weeks or even months to reap the restful benefits.

7. You're exercising too much
On the flip side, you can have too much of a good thing. If you're sweating every day or doing heavy-duty training for an event like a triathlon, fatigue and trouble sleeping can serve as a sign that you're pushing your body beyond its limits. Workouts--and especially endurance sports like long-distance running and cycling--also cause a spike in cortisol. If you're not striking the right balance between activity and rest, you can overload your system with physical stress just as you can with emotional or mental pressure.

Your fix: If you're exercising regularly but suddenly tire more easily, you may be overreaching. Try taking a few days of complete rest. Then ease back into your routine, doing about 25% of your usual activity for a week and adding another 25% each week until you're back up to speed.