The aims of this research were to examine the effects of infant feeding method on sleep development across the first twelve months of life, and to determine whether there are differences in sleep development between infants who are exclusively breastfed and those who are not. The participants were 52 infants and their parents. Parents completed sleep diaries for six consecutive nights once a month, for 12 months starting at one month of age, recording infant and parent sleep-related behaviours. It was predicted that infants who were breastfed for a longer period would have higher instances of night waking, would take longer to achieve the three criteria for sleeping through the night, and would have higher Composite Sleep Scores (Richman, 1981) than their non-breastfed counterparts. There was an unexpected high rate of breastfeeding in the sample of infants across the first 12 months. The breastfed infants displayed less night waking than their mixed or bottle-fed counterparts before three months of age, but more night waking after three months for the remainder of the study. They also took longer to reach each of the three sleeping through the night criteria. However, two subsets appeared within the breastfeeding group - those infants who experienced consolidated sleep earlier than six months of age, and those who did not. This finding demonstrated that breastfed infants are capable of sleeping through the night from an early age, contrary to what previous literature suggests.