1. A method for determining a health condition, the method comprising: receiving, by a health monitoring computing device, a first oxygen saturation level and a second oxygen saturation level from at least two sensors disposed proximate a subject; determining, by the health monitoring computing device, a level of cerebral extraction of oxygen using the first and second oxygen saturation levels; comparing, by the health monitoring computing device, the level of cerebral extraction of oxygen with thresholds, wherein the thresholds are personalized for the subject based on the level of cerebral extraction of oxygen of the subject during each of a resting state, a normal state and a restlessness state of the subject over a period of time; and identifying and outputting, by the health monitoring computing device, a mental fatigue level of the subject based on the comparison, wherein the mental fatigue level indicates the health condition of the subject.