Background: Heavy wrapping and head covering are risk factors for Sudden Infant Death Syndrome. A new mattress construction, PurFlo®, has extremely low thermal resistance, and when used with an infant sleeping bag minimizes the risk of head covering. Aim: To investigate the thermal balance and metabolic rate of infants sleeping on a conventional mattress or a Purflo® mattress in infant sleeping bags. Methods: A longitudinal study of thermal balance of infants during day‐time sleep on both mattress types in thermoneutral and cool conditions at ages 3 weeks (n = 24), 3 months (n = 22) and 5 months (n = 18). Results: In thermoneutral conditions axillary temperatures in quiet sleep were lower on the conventional mattress than on the PurFlo® mattress (p < 0.05, Wilcoxon test). On lowering room temperature to 15–16°C axillary temperatures fell, particularly in the older babies, and at each age were lower on the conventional mattress than the Purflo® (differences 0.14–0.72°C, p < 0.05, Wilcoxon test). Conclusion: In both thermoneutral and cool conditions, infant temperatures were higher on the PurFlo® than the conventional mattress. The more deformable surface of the PurFlo® mattress may lead to more effective insulation by the sleeping bag despite a lack of mattress insulation.