 This study will investigate the spatio-temporal characteristics of brain activity during sleep. Functional magnetic resonance imaging fMRI studies have shown that in the absence of external stimuli, the brain continues to show spatial patterns of activity that resemble those during sensory and cognitive tasks. This phenomenon greatly affects the interpretation of neuroimaging studies based on positive emission tomography PET and fMRI, which rely on the contrast between brain activity during a task and activity during rest. In addition, resting state activity in itself may reveal information on the large-scale organization of neuronal networks and on functional abnormalities related to disease. 