noun
1. : a serious disease prevalent in much of tropical Africa that is characterized by fever, protracted lethargy, tremors, and loss of weight, is caused by either of two trypanosomes ( Trypanosoma gambiense and T. rhodesiense ), and is transmitted by tsetse flies — called also African sleeping sickness, African trypanosomiasis
2. : any of various viral encephalitides or encephalomyelitides of which lethargy or somnolence is a prominent feature ; especially : equine encephalomyelitis
3. also sleeping disease : sleepy disease