The cause of fatigue is not always obvious (see Figure 4). If you have the flu, you expect to feel tired. Or you may want to nap after a big lunch. But you may feel inexplicably groggy throughout the day, not just during the biological trough that normally occurs in mid-afternoon. Your doctor may find nothing wrong, leaving you to wonder if your symptoms are “all in your head.”
Some causes of fatigue are obvious, such as lack of sleep or a medical illness, but many others are harder to pinpoint. Depression or anxiety, overwork, sedentary living, nutritional factors, even a medication could contribute to fatigue or cause a feeling of low energy.
The underlying cause of persistent fatigue can be hard to identify, but that doesn’t mean the fatigue is imaginary. Fatigue often signals that something is wrong, physically or emotionally, or even with the pace and tempo of your daily schedule. The following are the most common causes of an ongoing lack of energy.
Stress
Picture yourself in a stressful situation. You’re at home waiting for your doctor to call with some test results. You can’t stop thinking about the possibilities: What are the odds that the results will indicate a health problem? What will you do? You’re nervous and anxious. To calm down, paradoxically, you keep busy — you clean out a closet, organize your desk, weed the garden. You’ve got energy to burn.
Stressful events and the emotions they elicit give you a surge of energy, but only in the short term. In such situations, the body churns out stress hormones such as cortisol, epinephrine, and norepinephrine (see Figure 5). In response, blood flow increases to the brain and heart. Your heart pumps faster, your blood pressure goes up, and you take deeper breaths. This is the fight-or-flight response, a survival mechanism that evolved hundreds of thousands of years ago to protect our early ancestors in life-threatening situations. The same response that told them to run from a dangerous wild animal tells us to flee a burning building or dodge a speeding car.
Something stressful, such as the sight of a bus barreling toward you, causes a quick signal from your brain’s hypothalamic-pituitary system to the adrenal glands, which release the stress hormone epinephrine. This chemical speeds up your heart rate and breathing, and floods the bloodstream with glucose for energy so you can respond quickly. Other systems that are not needed for a quick response, such as the digestive and reproductive systems, temporarily shut down. The adrenal glands also slowly release glucocorticoids, which play a role in chronic stress.
But the fight-or-flight response guzzles energy, which is why it’s natural to feel fatigued after a stressful event. If you’re able to take it easy afterward, you’ll catch your breath and regain your energy. But chronic stress is far more harmful. For example, caring for a relative with a debilitating, long-term illness leaves you no chance to regroup. Instead, the stress saps your energy over time, leaving you feeling worn out (see “Caring for a family member”). Job pressures, turmoil in your relationships, and worrying about money are among the many other ongoing circumstances that can keep the stress hormones flowing.
People who regularly feel fatigued don’t necessarily have more stress in their lives than others. What is different is that their response to stress is more emotional. In response to the sorts of stressful events that everyone encounters — a rude remark by a store clerk, a traffic jam, a pending deadline — they get angrier, more hostile, more anxious, more nervous, and so on. To some degree, your response to stress is genetic, but it’s also learned. The good news is that you can also learn to moderate your response with methods such as relaxation techniques or cognitive behavioral therapy and, in so doing, boost your energy level over the long haul (see “Control stress”).
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