You stayed up late binge-watching Homeland. Then you woke up extra early to beat the boss to the office. Some days, there’s no mystery as to why you need an extra shot of espresso (and if all you need is more rest, power down your DVR and check out our guide to alternative sleep remedies.)

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. “It’s like asking a pediatrician why a baby is crying; the answer could be any number of things,” said Dr. Tanvir Hussain, a preventive cardiologist in Los Angeles.

A challenging puzzle, yes, but your fatigue is a mystery you can solve. Here are seven reasons you could be dragging—and how to regain more energy than you ever remember having.

You’re dehydrated

Healthy women who failed to replace a mere 1.5 percent of their water weight experienced mood swings and low energy levels, according to a 2012 study in The Journal of Nutrition. 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.

Your fix: Drink up, and and ditch that eight-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, says Gina Sirchio, a chiropractic physician and nutritionist at the LaGrange Institute of Health in Chicago.

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, as do people who’ve had stomach or intestinal surgeries (these procedures often alter the tissue where B12 absorption takes place, Sirchio said). Even low or borderline levels—not necessarily full-blown deficiency—can wear you down.

Your fix: 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," Sirchio said—it’s easier for your body to use. 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.

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You’re overwhelmed with stress

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, says Marc Bubbs, founder of Naturopathic Sports Medicine in Toronto. 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. Try this mini-meditation from Kathy Gruver, author of Conquer your Stress with Mind/Body Techniques: Concentrate on your breath (don’t change it—just observe it). On each inhale, think “I am.” On each exhale, “at peace.” Repeat it for a few minutes while you’re waiting in line or make it a full 15-minute meditation session.

You have hidden heart disease

In a study in the journal Heart & Lung, 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, Hussain said. 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, Hussain said.

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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 formed red 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, said Sirchio.

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, Sirchio warned—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.

You’re not working out

Especially when paired with chronic stress, too much time spent sedentary drains your fuel tank even though you’re merely idling, Bubbs said. Picture it: A stressful day at work cranks up your cortisol and blood 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 fatigue and more energy and vigor than those who don’t, according to a recent study in Medicine & Science in Sports & Exercise. If you’re using exercise to help you sleep better, give your new regimen time to take hold—another study in the Journal of Clinical Sleep Medicine found you’ll need to move consistently for a few weeks or even months to reap the restful benefits.

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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, Bubbs pointed out.

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 percent of your usual activity for a week and adding another 25 percent each week until you’re back up to speed, advised Tom Holland, 21-time Ironman Triathlete and author of The Marathon Method.

You have a urinary tract infection

If you’ve had a UTI before, you know the burning urgency that comes when you pee. But about half of women who show up with UTIs also report fatigue and a general sense of illness, and the rate increases among those 40 or older, said Dr. Ashley Carroll an assistant professor of urogynecology at Virginia Commonwealth University. “Basically, it's your body’s way of forcing you to rest in order to focus energy on fighting the infection,” Carroll said.

Your fix: Head to the doctor if you suspect a UTI. Prescription antibiotics can banish the bacteria. All your symptoms, including fatigue, should subside within seven to 10 days of completing treatment. As you’re healing, get plenty of rest, stay hydrated, and eat a healthy diet, Carroll advised. If you’re prone to frequent UTIs (more than a couple per year) talk with your doctor—long-term prophylactic antibiotics can ward off future infections.