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.