A system for measuring and monitoring physiologically relevant motion of a subject includes at least a motion sensor to measure movement of the subject and produce a series of movement data representing the movement of the subject over a period of time. The system also includes at least a biometric sensor to simultaneously measure biometrics of the subject and produce a series of biometric values of the subject over the period of time. The system is configured to determine a noise-to-signal ratio for the series of movement data as a function of biometric intervals in the series of biometric values and identify at least a portion of the series of movement data as corresponding to a physiologically relevant biorhythm. The system can be used to diagnose and monitor a disease or disorder, including a neurological disorder or a traumatic brain injury.
