1. A method for determining a respiration rate from a photoplethysmograph (PPG) signal, the method comprising: generating a plurality of morphology metric signals based on the PPG signal; generating an autocorrelation sequence based on the plurality of morphology metric signals; determining, using processing equipment, a plurality of peak values based on the autocorrelation sequence; accessing, using the processing equipment, a historical distribution of the respiration rate; and for each of the plurality of peak values: associating, using the processing equipment, the peak value with a first portion of the historical distribution of the respiration rate; and adjusting, using the processing equipment, the peak value based on the first portion of the historical distribution of the respiration rate; and calculating, using processing equipment, an updated value of the respiration rate based on the adjusted plurality of peak values.