The purpose of this study is to define human milk (HM) as an ecosystem which investigators will then combine into temporal models of milk dynamics to accurately describe HM chronobiology. This study addresses 4 crucial public health gaps: 1) how breast milk changes over time and over the day, 2) how milk dynamics are related to infant sleep patterns, 3) how milk dynamics are related to infant microbiome dynamics, and 4) how all these relationships differ between infants fed directly at-the-breast vs pumped milk. These fundamental insights have been unknown until now, so that families who feed pumped breast milk are completely underserved. These results are critical to optimizing infant feeding and health outcomes for all infants receiving breast milk.

Inclusion Criteria:

• Lactating mothers between 20-40 years old at enrollment

  o 120 mother/infant dyads who primarily feed HM ATB (average >75% of feeds,

• pumping no more than one time per day)

  o 120 mother/infant dyads who predominantly feed expressed HM (average >75% of feeds)

• All races and ethnicities may enroll

• Singleton infant delivered after 37 weeks

• Infant between the ages of 0-1 months at the time of enrollment.

• No serious health complications in mother or infant

Exclusion Criteria:

• Non-English-speaking subjects as study personnel only speak English
• Lactating moms who will be feeding both ATB and expressed HM (over 25% of the time in either feeding mode)
• Infant supplementation with formula ≥10 oz before sample collections begin, and/or no more than 16 oz during active participation/ sample collections.

240 mother/infant dyads exclusively feeding HM to healthy infants between 1-5 months postpartum: 120 mothers primarily feeding ATB (ATB Group) and 120 mothers predominantly feeding expressed HM (Express Group)