🫀How stress is calculated

The link between psychological stress and reduced heart rate variability has meta-analytic support: a systematic review and meta-analysis of short-term HRV found time-domain features distinguish states of acute mental stress, and a separate meta-analysis reports the same consistent association between stress and lowered HRV.

We use time-domain features ONLY. Frequency-domain features — the LF/HF ratio above all — are often used to infer "sympathetic-parasympathetic balance", but that interpretation is directly disputed: a dedicated paper argues LF/HF does not accurately measure that balance. So we do not use it.

Some products claim to identify discrete emotions from HRV — pleased, anxious, focused. We do not, because the evidence does not support that resolution: HRV indexes AROUSAL, not the VALENCE of an emotion. High arousal can be excitement or anxiety, and HRV cannot separate them.

What the literature does support is two weaker things: people with anxiety disorders show lower HRV than controls (a between-group difference, not a moment-to-moment reading), and HRV indexes emotional regulation capacity (a trait, not a current feeling). Getting from there to "six named emotions" goes beyond the evidence.

One elevated day is common: hard training, short sleep, alcohol and the onset of a cold all suppress HRV. What is worth noticing is a RUN of elevated days — that is the accumulation signal.

So Vita only notifies after three or more consecutive elevated days, and then stays quiet for five. Being told you are stressed is itself a stressor, and an alert that fires often is the one people switch off.