Despite its prevalence and consequences, the pathophysiology of insomnia and, specifically, the pathway by which morbidity risk is conferred, has been relatively unstudied. With respect to medical illness in particular, insomnia may confer risk in several ways, including: 1) an inherent compromise in the restorative/conservative function of sleep, 2) the deleterious effects of "hyperarousal" and/or HPA axis abnormalities on end organ integrity and function, and/or 3) diminished immunocompetence. This study focuses on the last of these possibilities, the relationship between immune function and sleep.