Models of the pathophysiology of human sleep disorders have only recently been tested using nuclear medicine assessments, which have greatly increased our understanding of the brain mechanisms involved in the human sleep-wake cycle. Dramatic changes in function have been observed in large-scale neuronal networks during sleep. Broad declines in heteromodal-association-cortical function, and relative increases in limbic and paralimbic function have been observed. These cortical areas are responsible for essential aspects of human behavior, allowing us to interact with the world around us and to evaluate the significance of important events in our lives. Preliminary findings suggest that fundamental alterations in the function of these neural systems occur in sleep disorders. In depression, alterations in rapid-eye-movement and slow-wave sleep appear linked to a sleep-related dysfunctional arousal in primary limbic and paralimbic structures (amygdala), and hypofunction in frontal cortical areas. Pharmacologic interventions partially reverse these alterations. Preliminary studies in insomia indicate a subcortical hyperarousal and a failure of sleep to provide normal restoration of function in the prefrontal cortex, leading to chronic sleep deprivation. This review discusses functional neuroimaging data on normal sleep, and on the pathophysiology of insomnia related to depression and primary insomnia.