There is disclosed a respiration rate monitoring system and method using a non-invasive device and method of monitoring it nearly continuously. One aspect includes receiving a series of signal data points from an optical sensor, detecting a series of peaks or valleys for the filtered signal data points, determining the time difference between individual peaks or valleys of the series of peaks or valleys to produce a series of time difference values, detecting peaks or valleys for the series of time difference values, determining widths between the peaks or valleys of the series of time difference values, and estimating a respiration rate from the widths between the peaks or valleys of the series of time difference values, and converting the estimation of a respiration rate to an indicator indicative of a respiration rate.
