Good sleep is necessary for both physical and mental health; sleep and immune responses are reciprocally and closely linked. Sleep loss impairs the immune response, while, on the other hand, the immune response, activated for instance by an infection, alters sleep. Sleep alterations induced by immune activation are mediated by cytokines such as interleukin-1. In the past, it was thought that cytokines were produced only by the immune system, and active only there as signaling molecules. Today it is clear that IL-1 and other cytokines are present and active in the healthy brain, where they physiologically interact with the brain circuits and the neurotransmitter systems (for instance the serotonergic, GABAergic, and cholinergic systems) that control sleep. These interactions are altered by immune response, and, as a result, non-rapid eye movement (NREM) sleep is increased and fragmented, whereas rapid eye movements (REM) sleep is inhibited.