Table 5. Multiple linear regression analysis between SDNNIDX and arousal index, TST90, and AHI not due to arousals.

SDNNIDX, ms	Regression coefficient	Confidence interval	P value
Arousal index, n/h	0.37	0.08 to 0.65	0.011
TST90, %	0.30	−0.10 to 0.70	0.141
AHI, n/h	0.03	−0.20 to 0.27	0.768
Age, y	−0.31	−0.57 to −0.05	0.018
Female sex	−4.68	−9.74 to 0.37	0.069
Beta-blocker therapy	−8.66	−19.75 to 2.43	0.125
Cups of coffee before bedtime, n	5.99	0.18 to 11.80	0.043
Heart rate, n/min	−1.25	−1.55 to −0.95	<0.001

Adjusted R²=0.30. SDNNIDX, mean of the standard deviations of normal sinus-to-normal sinus interbeat (NN) intervals in all 5-minute segments; TST90, total sleep time with peripheral oxygen saturation below 90%; AHI, apnoea-hypopnoea index with hypopnoeas based on 3%-oxygen desaturations only (not on arousals).