To test our hypotheses, we will: (1) examine expressed rhythmicity in children with ONH. These studies will use actigraphy, sleep questionnaires, and assessment of melatonin secretory profiles. (2) We will correlate hypothalamic anatomical abnormalities and the degree endocrine dysfunction with sleep and expressed rhythmicity. (3) We will test if short-term administration of melatonin improves sleep-wake patterns in children with abnormally-expressed rhythmicity.