Hypersomnia describes a symptom where the patient sleeps more than sixteen hours a day. Very few diseases cause this symptom. Those that do have a well established medical history and can usually be ruled out for a patient, such as African trypanosomiasis (which requires a history of travel in Africa) and obesity hypoventilation syndrome (which only occurs in the morbidly obese). Any other conditions that cause this symptom tend to be very rare.

Hypersomnia at Wikipedia

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