1. A method for using a functional near-infrared signal to correlate to a state of anesthesia, the method comprising: receiving the functional near-infrared signal in response to directing wavelengths of light in a near-infrared range on a patient; determining, from the functional near-infrared signal, a relative deoxyhemoglobin concentration and oxyhemoglobin concentration; and correlating the percentage of the deoxyhemoglobin concentration to the state of anesthesia.