STUDY OBJECTIVES: The prone sleeping position is a major risk for the sudden infant death syndrome (SIDS) and has been associated with lowered blood pressure and impaired blood pressure control. This study aimed to assess the effects of sleeping position, sleep state, and postnatal age on baroreflex control of heart rate. PARTICIPANTS: Term infants (n = 31) were studied at 2-4 weeks, 2-3 months, and 5-6 months with daytime polysomnography. INTERVENTIONS: Blood pressure and heart rate were recorded during quiet (QS) and active (AS) sleep in both the supine and prone positions. In each condition, three 1-2 minute baseline measurements and three 15 degrees head-up tilts were performed. MEASUREMENTS AND RESULTS: Baroreflex sensitivity (BRS) was assessed using cross-spectral analysis (BRS(SP)) and sequence analysis (BRS(SEQ)) in the baseline condition and with BRS(SP) during head-up tilting (BRS(SP) Tilt). BRS was usually lower prone compared to supine, reaching significance at 2-3 months (BRS(SP), P <0.05; BRS(SP) Tilt, P <0.05) and 5-6 months (BRS(SEQ), P <0.05). BRS was lower in AS than QS supine at 5-6 months for all BRS estimates (P <0.05). During QS, BRS increased with postnatal age in both sleeping positions (P <0.05 for all BRS estimates); during AS, the postnatal age-related increase was limited to the prone position (BRS(SEQ), P <0.05). CONCLUSIONS: Sleeping position, sleep state and postnatal age all affect infant baroreflex function. Reduced BRS in the younger infants sleeping prone could increase the vulnerability to hypotensive events during sleep and thus play a vital role in conditions where circulatory failure may be involved, such as SIDS. CITATION: Yiallourou SR; Sands SA; Walker AM; Horne RSC. Baroreflex sensitivity during sleep in infants: impact of sleeping position and sleep state. SLEEP 2011;34(6):725-732.