Sleeping sickness, once under control, is a re-emergent endemic parasitic disease in intertropical Africa. Its originality resides in its duality. Two trypanosome groups (Trypanososma brucei gambiense vs.rhodesiense ) are transmitted to humans by tsetse flies from two geographical areas (Western and Central Africa humid forest vs. Eastern Africa arboreous savannah), provoking a slowly or a rapidly evolutive disease. The two stage (haemolymphatic vs. neurological invasion) pathogenic evolution leads to the duality of the immune response, depending on the host-parasite inter-relation differences in the blood and the brain. In the blood, the immune processes involved are both specific (anti-variant surface glycoprotein (VSG) antibodies) and non-specific (complement-mediated lysis, opsonification-facilitated phagocytosis and antibody dependent cell-mediated cytotoxicity). Although macrophages are activated in the blood and infiltrate the brain, nitric oxide decreases in the blood and increases in the brain, with a breakage in the blood-brain barrier, leading to brain lesions through the production of deleterious molecules. Prophylactic means are affected by the duality of pathogenic processes. This finally leads to a two stage disease (haemolymphatic vs. neurological) with two different therapeutic strategies. The sleep-wake cycle and other biological rhythms are also marked by the disappearance of circadian rhythmicity demasking basic ultradian activities and relationships, such as the interdependence of endocrine profiles and the sleep-wake alternation. 2001 Harcourt Publishers Ltd