Feels like...you're inexplicably drained of vigor despite getting enough sleep.
The Cause: You may be suffering from a form of sleep apnea, in which your airway becomes temporarily blocked while you're sleeping, leading to pauses between breaths, a disrupted sleep cycle, and less time in deep sleep. But you may have a more serious condition such as thyroid dysfunction (women are more likely than men to have thyroid problems). The thyroid controls everything from metabolism to body temperature, so misfires of the gland have wide-reaching effects, including fatigue. Another potential trigger: anemia, a deficiency in red blood cells or hemoglobin (which carries oxygen in your blood), that affects more than three million people. When body tissue doesn't get enough oxygen, you can feel light-headed, tired, and drained of energy.
The Cure: Schedule a checkup with your doctor if you feel bone-tired even after getting sufficient sleep and sticking to a decent diet. "So many people write off fatigue as simply a by-product of working too much, but it can be a symptom of larger, more serious health issues that can usually be resolved quickly once diagnosed," says Breus.
Feels like...full-body fatigue.
The Cause: Working your body in ways it's not used to. Overexerting yourself at the gym, doing heavy lifting around the house, even walking all day on vacation may cause microscopic tears in your muscles, and as they repair, you can feel a reduction of strength for days.
The Cure: Give your body the break it needs to recover. Put off any activity that may be physically taxing for at least 24 hours or until muscle soreness eases.
Next: Why you're tired after lunch
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