Objective. Prone sleeping position has a strong link to sudden infant death syndrome (SIDS), and the “Back to Sleep” campaign has played an important role in reducing SIDS. We tested the hypothesis that the mechanism of the sleep position effect is based on changes in sleep, arousal, heart rate variability (HRV), and the QT interval of the electrocardiogram.

Study Design. We studied 16 premature infants longitudinally, at 1 and 3 months’ corrected age. Videosomnography recordings were made during the infants’ normal daytime naps. Each infant was recorded in both supine and prone positions. The recordings were analyzed in 30-second epochs, which were classified as awake, active sleep (AS), quiet sleep (QS), or indeterminate sleep. Electrocardiogram data were sampled with an accuracy of 1 millisecond. Time domain analysis of HRV was measured by standard deviation of all R-R intervals and by the square root of the mean of the sum of the squares of the differences between adjacent R-R intervals. Frequency domain analysis was done for low frequency (0.04–0.14 Hz) and high frequency (0.15–0.5 Hz) HRV. We measured QT, JT, and R-R intervals during AS and QS for each position.

Results. We found no significant differences between supine and prone position, either in total sleep time or in percentage of QS. Percentage of AS was significantly lower in the supine position, but only at 1 month corrected age. The incidence of short, spontaneous, sleep transitions was significantly higher in supine, also only at 1 month corrected age. Time domain analysis of HRV showed a significantly lower variability in prone, but only during QS. Frequency domain analysis of HRV showed no differences between the 2 sleeping positions. Both QT and JT intervals were significantly longer in prone during QS, but only at 1 month corrected age.

Conclusions. Despite the commonly held belief, prone position did not substantially increase total sleep at these ages. On the other hand, prone sleeping decreased the number of sleep transitions at 1 month corrected age, increased QT and JT intervals, and reduced HRV, thereby potentially increasing the vulnerability for SIDS. This study supports “Back to Sleep” as the position of choice not only for term but also for preterm infants after discharge home.

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