The hypocretins (orexins) are a newly identified (1998) peptide family comprised of two peptides, hypocretin-1 and hypocretin-2. Recent observations suggest an involvement of these peptides, located primarily in the hypothalmus, in the regulation of behavioral state. They are involved in circadian rhythms, metabolism, hormone secretion, body temperature, appetite, cardiovascular functions (e.g., blood pressure), and sleep abnormalities (e.g., narcolepsy and cataplexy).