This project explores the potential of a novel approach to functional studies of the brain with near-infrared spectroscopy. We hypothesize that the phase relationship between oscillations of cerebral oxy-hemoglobin and deoxy-hemoglobin concentrations provide information about (1) the interplay of hemodynamic and oxygenation-related processes that are associated with brain activation, and (2) functional connectivity networks involving distinct cortical areas. Specifically, in this project we will perform a new analysis of data collected previously in a full-night sleep study using a novel phasor-based analysis and representation of hemodynamic oscillations. This new analysis will allow us to test the hypothesis that hemodynamic conditions associated with different sleep stages induce different phase relationship between spontaneous oscillations of oxy-hemoglobin and deoxy-hemoglobin concentrations at frequencies around 0.1 Hz (Mayer waves). We will also test the hypothesis that oxy-hemoglobin and deoxy-hemoglobin concentration changes have different relative phases depending on the cortical area activated and/or the stimulation protocol. Finally, we will perform pilot measurements to test the potential of this new phase-based method in studying functional connectivity networks in the brain. This project can lead to a novel approach to functional brain studies, and explores signals that are not accessible with functional magnetic resonance imaging, involving both paramagnetic and diamagnetic hemoglobin species.