Lethargy is a state of being exhausted, with little to no energy, and can refer to feelings of fatigue as well as a lack of mental or physical motivation. It can be a sign of an underlying physical or mental condition, and can cause changes in mood, decreased alertness or decreased ability to think, fatigue, low energy, and sluggishness. Lethargy is different from just being drowsy or sleepy, and indicates something is affecting the brain, level of consciousness, and other mental functions. Some causes of lethargy include overexertion, not getting enough sleep, the flu or a stomach virus, carbon monoxide poisoning, dehydration, fever, hyperthyroidism, hypothyroidism, hydrocephalus or brain swelling, kidney failure, Lyme disease, meningitis, pituitary diseases, such as pituitary cancer, nutrition deficiencies, sleep apnea, stroke, and traumatic brain injury. Treatment for chronic lethargy linked to an underlying health condition will focus on treating the condition to see an improvement in symptoms. If lethargy is accompanied by noticeable, marked changes in behavior, it is often cause for concern and immediate medical attention should be sought.