An answer to the question as to why we sleep may be found in a recent review article in the Journal of Allergy and Clinical Immunology (December 2005). Individuals know intuitively that they sleep more when they are sick and that they need a certain amount of sleep to stay healthy. The findings summarized in this article, gathered over the last 25 years, help to explain whya link between the innate immune system and the regulation of sleep. Both ordinary sleep and sleep associated with an infection appear to be regulated by one or more of the >100 cytokines synthesized by immune cells. The answer to the question about why individuals sleep may be, in part, to maintain their immune function.