Thanks for pointing out that we should make any causal links more clear. We added the following information:

Active sleeping is a condition associated with more heart rate irregularities and sleep apnea which are potentially life-threatening if the baby does not arouse). When babies are actively sleeping, breastfed babies are more easily aroused than formula-fed babies (Horne et al 2004; Franco et al 2000). Arousability is related to risk for SIDS (Kato et al., 2003).

Franco P, Scaillet S, Wemenbol V, Valente F, Grosswasser J, and Kahn A. 2000. The influence of a pacifier on infants’ arousals from sleep. J Pediatr 136: 775-779.

Horne RSC, Parslow PM, Ferens D, Watts AM, and Adamson TM. 2004. Comparison of evoked arousability in breast and formula fed infants. Arch Dis Child 89: 22-25.

Kato I, Scaillet S, Groswasser J, Montemitro E, Togari H, Lin JS, Kahn A, and Franco P. 2006. Spontaneous arousability in prone and supine position in healthy infants.Sleep 29(6):785-90.