OBJECTIVE: To investigate whether sleep disturbances during pregnancy affect cardiac autonomic modulation in the resting state. METHODS: A cross-sectional study was conducted of 160 pregnant women at various stages of gestation. Participants were interviewed about their sleep length per night, sleep quality (Pittsburgh Sleep Quality Index, PSQI), daytime sleepiness (Epworth Sleepiness Scale, ESS), snoring habits, and symptoms of restless legs syndrome during the previous few weeks. Cardiac autonomic modulation in the resting state was quantified by spectral analysis of heart rate variability (HRV) from short-term electrocardiogram monitoring. The relationship of HRV to diverse covariates was studied by multiple regression analysis. RESULTS: No significant influences were observed of short sleep duration (<7 hours per night), poor sleep quality (PSQI score >5), or restless legs syndrome on HRV measures. Participants with excessive daytime sleepiness (ESS score >9) and habitual snorers had a significantly elevated low frequency/high frequency (LF/HF) ratio and LF power in normalized units (LF norm). In multiple regression analyses, habitual snoring was strongly and positively associated with LF/HF ratio (P<0.001) and LF norm (P<0.001). CONCLUSION: Resting cardiac autonomic modulation was found to shift toward a sympathetic predominant state among pregnant women who are habitual snorers.