Lethargy, weariness, lassitude. These are terms that are commonly used to describe a lack of energy. It's normal to feel run down and in need of a nap every once and awhile. Some people however, feel this way on a regular basis. Doctors often refer to recurring feelings of low energy as fatigue.

Fatigue can be related to underlying medical conditions, poor nutrition and stress, as well as other factors. While it can often be difficult to discern the root cause of fatigue, it is commonly a symptom of disorders such as fibromyalgia, chronic fatigue syndrome, and hormonal disorders such as Addison's disease and hypothyroidism. Some specific forms of fatigue involving loss of muscle strength may be associated with kidney, liver, heart, and lung disease. Often however, it can be difficult to distinguish fatigue that is a result of these disorders and fatigue that arises from a simple lack of sleep. This may be particularly true when an underlying disorder results in chronic sleep disruption or in abnormal sleep.