Background: The postpartum experience of preterm mothers is unlike the biological norm, and their sleep experience is poorly understood. Preterm mothers are physically separated from their infants, and few report waking through the night to pump breast milk, which may contribute to breast milk insufficiency. Breast milk insufficiency is common, and many preterm mothers cannot pump enough milk to meet their infant's needs. Despite this common obstacle, and the benefits of sleep to overall health, the relationship between objective maternal sleep measures and subsequent milk volume has never been explored. When a preterm mother experiences milk insufficiency, donor milk is prescribed; however, pasteurized donor milk contains no commensal bacteria, which are associated with improved neonatal outcomes. Milk microbiota profiles are highly variable between women, and some profiles are more protective. Factors that influence milk microbiota are poorly understood, though emerging evidence suggests a diurnal influence. Subject Population: Breastfeeding mothers with hospitalized preterm infants. Design: Maternal sleep characteristics and milk profiles (microbiota and volume) will be examined in this longitudinal, prospective, observational study. Methods: Subjects will wear an Actiwatch 2 (Philips Respironics) and complete daily sleep and pumping logs before discharge from the hospital. Once breast milk has come in and for one week, two daily breast milk samples (one daytime, one nighttime) will be collected. Bacterial DNA will be extracted from breast milk samples, 16S rRNA PCR amplification will be performed, and amplicons will be sequenced using Illumina MiSeq. Analysis: Group-based trajectory analysis will be applied to identify distinct sleep and milk profile patterns. Relevance to NINR: Neonatal Intensive Care nurses are directly involved in both: 1) the support of new mothers (including sleep hygiene), and 2) breast milk administration (tracking, storage, measurement, fortification, and preparation). This study will inform future strategies related to sleep support and personalized breast milk for preterm infants.