Interactions between sleep and the immune system have been recognized for millennia. The lethargy and increased desire to sleep that accompany mild infections such as colds or "the flu" are common experiences. These experiences have fostered the belief that sleep promotes recovery from infectious challenge. Another common belief is that the lack of sleep increases susceptibility to infectious disease. However, despite these age-old and widespread beliefs, surprisingly little empirical evidence supports the hypotheses that increased sleep aids recovery from, and lack of sleep increases susceptibility to, infections. Although research conducted over the last 30 years has clearly demonstrated that sleep is altered during the course of infection, few experiments have directly tested the functional impact of sleep on responses to immune challenge. We will review relevant literature documenting that sleep patterns do indeed change during states of infectious disease, discuss potential mediators of these alterations in behavior, and finally address the issue of whether sleep or sleep loss impacts the ability of the host to mount an appropriate immune response.