OBJECTIVES:Many studies of infant sleep rely solely on parentally-reported data, assuming that parents accurately report their infant's sleep parameters. The objective of this paper is to examine whether night-time sleep parameters of exclusively breastfed or exclusively formula-fed infants differ, and whether correspondence between parental reports and objective measures varies by feeding type. METHODS:Mother-infant dyads intending to breastfeed or formula-feed exclusively for 18 weeks were recruited. Mothers were multiparas and primiparas, aged between 18 and 45 years. Infants were full-term, normal birthweight singletons. Maternal report and actigraphic data on infant sleep were collected fortnightly, from four to 18 weeks postpartum. Data were analysed cross-sectionally using t-tests and GLM analysis to control for interaction between feed-type and sleep location. RESULTS:Actigraphy-assessed infant sleep parameters did not vary by feed-type but parentally reported sleep parameters did. Maternal report and actigraphy data diverged at 10 weeks postpartum and discrepancies were associated with infant feeding type. Compared to actigraphy, maternal reports by formula-feeding mothers (controlling for infant sleep location) over-estimated infant's Total Sleep Time (TST) at 10 weeks and Longest Sleep Period (LSP) at 10, 12 and 18 weeks. CONCLUSIONS:These results raise questions about the outcomes of previous infant sleep studies where accuracy of parentally-reported infant sleep data is assumed. That parental reports of infant sleep vary by feeding type is particularly important for reconsidering previous studies of infant sleep development and intervention studies designed to influence sleep outcomes, especially where feed-type was heterogeneous, but was not considered as an independent variable.