 Objective. The objective of the proposed study is to investigate the spatio-temporal characteristics of brain activity during sleep. A number of recent fMRI Ogawa, Lee et al. 1990 studies have shown that in absence of external stimuli, the brain continues to show spatial patterns of activity that resemble the networks that activate during sensory and cognitive tasks. This phenomenon greatly affects the interpretation of neuroimaging studies based on PET and fMRI, since these rely on differential imaging in which activity during a task is contrasted against activity during rest. In addition, resting state activity in itself potentially contains unique information on the large scale organization of neuronal networks and reveals information about functional abnormalities related to disease processes. 