Table 3. Multiple linear regression analysis between SDNNIDX and TST90.

SDNNIDX, ms	Regression coefficient	Confidence Interval	P value
TST90, %	0.40	0.04 to 0.76	0.029
Age, y	−0.32	−0.58 to −0.06	0.017
Female sex	−4.81	−9.85 to 0.24	0.062
Beta-blocker therapy	−9.18	−20.28 to 1.92	0.105
Cups of coffee before bedtime, n	5.36	−0.46 to 11.17	0.071
Heart rate, n/min	−1.27	−1.57 to −0.96	<0.001
Total sleep time, h	−0.04	−0.09 to 0.01	0.080

Adjusted R²=0.27. 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%. In this model, total sleep time has been added as a controlling variable since TST90 is not a frequency index.