An evolutionarily informed perspective on parent-infant sleep contact challenges recommendations regarding appropriate parent-infant sleep practices based on large epidemiological studies. In this study regularly bed-sharing parents and infants participated in an in-home video study of bed-sharing behaviour. Ten formula-feeding and ten breastfeeding families were filmed for 3 nights (adjustment, dyadic and triadic nights) for 8 hours per night. For breastfed infants, mother-infant orientation, sleep position, frequency of feeding, arousal and synchronous arousal were all consistent with previous sleep-lab studies of mother-infant bed-sharing behaviour, but significant differences were found between formula and breastfed infants. While breastfeeding mothers bed-shared with their infants in a characteristic manner that provided several safety benefits, formula-feeding mothers bed-shared in a more variable manner with consequences for infant safety. Paternal bed-sharing behaviour introduced further variability. Epidemiological case-control studies examining bed-sharing risks and benefits do not normally control for behavioural variables that an evolutionary viewpoint would deem crucial. This study demonstrates how parental behaviour affects the bed-sharing experience and indicates that cases and controls in epidemiological studies should be matched for behavioural, as well as socio-demographic, variables.