If you're not getting enough sleep, it stands to reason you'll be tired. But what if you don't know that you aren’t getting sufficient sleep? This is often the case with a condition called sleep apnea -- a  sleep disorder that causes you to momentarily stop breathing, often many times during the night. Each time you stop breathing, you awaken just long enough to disrupt your sleep cycle, usually without being aware of it. Your only clue, says Goldberg, is that you experience constant fatigue no matter how many hours you sleep each night.

According to Goldberg, sleep apnea, which is caused by an upper airway obstruction, often occurs in women who are overweight or obese. Snoring is often a sign of sleep apnea. Diagnosis requires a visit to a sleep lab, or to a doctor specializing in sleep apnea.

If you have sleep apnea, your physician will recommend lifestyle changes, including losing weight and quitting smoking. Medical treatment includes devices that keep airway passages open while you sleep. In extreme cases, surgery may be necessary to ensure proper airway flow.  Left untreated, sleep apnea can increase your risk of stroke or heart attack.

Fatigue Cause # 7: Undiagnosed Heart Disease