We calculate stress and energy based on the evaluation of heart rate variability (HRV). Heart rate variability shows the variation in the time intervals between heartbeats (RR intervals).
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For a person with a pulse of 60 beats per minute, the RR-intervals look something like this: 0.83 s., 1.31 s., 0.73 s., etc.

Sample of a cardiogram showing intervals between heartbeats in milliseconds