Background: Cranial radiation therapy (CRT) is required for successful treatment of a variety of brain tumours in childhood. Objective: To investigate whether childhood CRT leads to altered sleep -wakefulness organization in adulthood, and to identify the determinants of such alterations. Subjects and methods: Subjective (questionnaires) and objective (actigraphy) measures of circadian rhythmicity and sleep were assessed in 25 individuals, 8-29 years after CRT for medulloblastoma (n ¼ 17) or other intracranial tumours (n ¼ 8), and in a group of 34 age-matched healthy individuals. Serum GH peak during insulin-induced hypoglycaemia and serum concentrations of prolactin and leptin (expressed per fat mass) were determined in the CRT group. Results: The CRT group showed a markedly increased sleep duration (8.66 h, compared with 7.66 h in controls). In addition, the sleep -wake rhythm showed greater amplitude and less fragmentation, and less tolerance for alterations in the timing of sleep. Regression analysis showed both radiation dosage and neuroendocrine status to be determinants of sleep changes, suggesting that some of the alterations may be normalized with hormone supplementation.Conclusion: The present study shows that high-dose cranial radiation therapy in childhood is associated with objective and subjective changes in the sleep -wake rhythm in adulthood.