Effect of infant sleeping position on sleep spindles

Sleep spindles play an active role in inducing and maintaining sleep and may affect arousal by blocking the transmission of external stimuli through the thalamus to the cortex. Previously we have demonstrated that sleeping in the prone position impairs arousal in infants at 2-3 months of age, but not at 5-6 months. We aimed to examine if sleeping position and postnatal age affected duration and/or density of sleep spindles. Twenty-one healthy term infants were studied using daytime polysomnography at 2-3 months and 16 were again studied at 5-6 months. Infants slept both prone and supine at each study. The mean duration of non-rapid eye movement (NREM) sleep was not different between the two studies in either position. At 2-3 months both spindle density (P < 0.001) and proportion of NREM sleep (P < 0.025) with spindles were significantly greater in the supine than in the prone position. At 5-6 months spindle duration was longer in the supine than in the prone position (P < 0.03). Spindle density in the supine position was not different between the two studies, however, when infants slept prone, it was significantly increased at 5-6 months compared with 2-3 months (P < 0.001). Arousal threshold was not correlated with either spindle density or percentage of NREM sleep with spindles in either position at either study. In this study spindle density and the percentage time spent with spindles were not well correlated with infant arousability, and hence may not be able to be used as markers of depressed arousal responses in infants.