During the postpartum period, birthing parents’ sleep patterns are linked tightly to infant feeding. Public health agencies worldwide endorse exclusive breastfeeding as the preferred method for infant feeding for the first 6 months because of associated health benefits. Sleep behaviors and endogenous circadian rhythms influence lactation hormones, while at the same time, prolactin has been shown to increase slow-wave sleep. Many new parents add formula to their infants’ diets before 6 months, mistakenly believing that supplementation will help their baby sleep longer at night. However, there are negligible differences in sleep duration between breastfed and formula-fed infants. Observational studies show that bed-sharing between the mother–infant dyad is associated with longer breastfeeding duration. Nevertheless, room-sharing is recommended because bed-sharing increases the risk of sudden infant death syndrome and sleep-related infant death. Because bed-sharing is the preferred sleeping arrangement in some cultures and families, there are calls for more research on this controversial topic. Lactation and breastfeeding can be more complex in certain circumstances, for instance, in premature and hospitalized infants or when the breastfeeding parent takes medications. For parents who work outside the home, the end of parental leave can raise new challenges for breastfeeding and sleep. (Authors’ note: The language around sex and gender related to birthing, parenting, and breastfeeding is changing rapidly. The authors acknowledge that not everyone who lactates identifies as a woman and that using gender-inclusive language can decrease bias and reduce health inequities, e.g., some individuals prefer the term “chest feeding” rather than “breastfeeding” (Bartick et al., Breastfeed Med 16:587–90, 2021). Thus, while most research that we cite in this chapter has been performed using women as the default/assumed gender of female parents, we have endeavored to use language throughout this chapter that respects nonbinary and diverse gender identities.)