Table 1: Descriptions of HRV measures and their tendency under stress [¹²].

Domain	Measure	Description	Features trend under stress
Time-domain	AVNN	Average of NN intervals (ms)	↓
-	SDNN	Standard Deviation of all NN intervals (ms)	↓↑
-	RMSSD	Root mean square of differences of successive NN intervals (ms)	↓
-	pNN20	NN variations above 20 ms (%)	↓
-	pNN50	NN variations above 50 ms (%)	↓
Frequency-domain	LF/HF	Describes the ratio of Low Frequency and High Frequency power bands.	↑