1. A method comprising: receiving a near-infrared signal in response to directing wavelengths of light in a near-infrared range on a patient; determining, from the near-infrared signal, a relative deoxyhemoglobin concentration and oxyhemoglobin concentration; analyzing the percentage of the deoxyhemoglobin concentration to determine a corresponding state of anesthesia; and administering an amount of anesthetic according to the corresponding state of anesthesia.