Method 100 and device for quantifying heart rate variability coherence of a subject is disclosed herein. The method 100 comprises obtaining a bio-signal (such as a PPG signal) from the subject at 102 and deriving a time-domain heart rate variability signal from the bio-signal at 108. Further, at 112, the method 100 further includes correlating the time-domain heart rate variability signal with a sine wave representing a time domain reference heart rate variability signal to obtain a correlated heart rate variability signal, and this includes adjusting frequency of the sine wave and performing cross-correlation between the sine wave at each of the adjusted frequencies and the heart rate variability signal to obtain the correlated heart rate variability signal. Further, at 116, the method includes quantifying the heart rate variability coherence based on the correlated heart rate variability signal.
