Table 3.

Partial correlations (controlling for age and BMI) between sleep quality (as measured by the PSQI) and heart rate variability (n = 60).

Control Variables: Age and BMI		Sleep Latency	Daytime Dysfunction	Subjective Sleep Quality	Global Sleep Score
Stress VLF	Correlation	−0.143	−0.448	−0.361	−0.320
	Significance (2-tailed)	0.303	0.001 *	0.007 *	0.018 *
	df	52	52	52	52
Stress HF n.u.	Correlation	−0.334	−0.166	−0.255	−0.332
	Significance (2-tailed)	0.013 *	0.231	0.062	0.014 *
	df	52	52	52	52
Stress TP	Correlation	−0.184	−0.399	−0.332	−0.318
	Significance (2-tailed)	0.183	0.003 *	0.014 *	0.019 *
	df	52	52	52	52
Stress Ratio	Correlation	0.274	0.094	0.094	0.158
	Significance (2-tailed)	0.045 *	0.499	0.498	0.253
	df	52	52	52	52

* Statistical significance: p < 0.05. Key: BMI = body mass index; df = degrees of freedom; HF n.u. = high-frequency heart rate variability (normalised units); ratio = sympathovagal balance; TP = total power heart rate variability; VLF = very-low-frequency heart rate variability.