Table 1. Heart rate variability parameters during humming, physical activity, sleep, and stress.

*Indicates the statistical significance of the specific parameter and activity combination vs. other activities, as calculated using single-factor ANOVA followed by the paired t-test, p<0.05

RMSSD: the root mean square of successive differences between normal heartbeats; SDNN: standard deviation of normal-to-normal Interval: LF: low frequency; HF: high frequency: ANOVA: analysis of variance

Activity	Stress index (SI)	Heart rate	RMSSD	SDNN	Total power	LF/HF ratio
Humming	9.26 ±2.49*	82.54 ±7.92	41.40 ±29.37	61.83 ±24.68*	4318.82 ±3350.67*	13.52 ±19.17*
Physical activity	23.36 ±13.19	103.27 ±12.21	18.66 ±21.01	26.73 ±21.68	1003.84 ±1700.29	5.24 ±2.58
Sleep	10.94 ±3.47	70.35 ±7.19*	45.62 ±28.83	47.76 ±20.82	2750.97 ±2538.47	2.26 ±1.47
Stress	21.68 ±7.39	97.21 ±14.52	14.81 ±10.19	22.41 ±12.08	648.43 ±891.78	5.70 ±3.42