Algorithm 1.

A procedure for using the instantaneous amplitude as a stress metric

1: Bandpass-filter the NNI into two bands, the LF and the HF

2: Apply the Hilbert transform to LF and HF to generate analytical signals

3: From the complex-valued analytic signals, obtain the amplitude at every point in time, the iA

4: Divide signals into time windows, typically sliding windows with a length of 5 min and a 10 s increment

5: In every time window, exclude the 20% largest and smallest values, to remove outliers

6: Calculate the mean for every time window