Without understanding its functional significance, we spend a third of our lives sleeping. As neuroscience provides new advances into understanding human brain function, an improved assessment technology is required to characterize the functional neurophysiology of sleep. The fMRI and PET methods of neuroimaging are of limited utility in characterizing sleep organization because their measurement technologies (requiring head fixation) are incompatible with a normal night's sleep. We propose to develop the SOMNA (Sleep Organization and Microstates of Neural Arousal) assessment system for dense sensor array electroencephalographic studies of bleep. The improved spatial sampling of the 128-channel EEG will correct an important inadequacy of conventional EEG, thereby promising new insights into brain activity. The SOMNA system would provide advanced computerized assessment of sleep EEG and polysomnographic measures, and it would segment the BEG into microstates of arousal as well as traditional sleep stages. The system would advance the technology of clinical sleep laboratories, providing new methods of dense array EEG for studies of current density, electrical field animations, and MR-constrained source localization.