Table 2. Multiple linear regression analysis between SDNNIDX and the arousal index.

SDNNIDX, ms	Regression coefficient	Confidence interval	P value
Arousal Index, n/h	0.42	0.16 to 0.68	0.002
Age, y	−0.28	−0.53 to −0.02	0.036
Female sex	−5.06	−10.06 to −0.07	0.047
Beta-blocker therapy	−7.78	−18.82 to 3.25	0.166
Cups of coffee before bedtime, n	6.22	0.41 to 12.04	0.036
Heart rate, bpm	−1.21	−1.51 to −0.91	<0.001

Adjusted R²=0.29. SDNNIDX, mean of the standard deviations of normal sinus-to-normal sinus interbeat (NN) intervals in all 5-minute segments.