Nearly all humans have a day-night cycle, or circadian rhythm, including the fetus and full term babies. However, the preterm baby has been shown to lack this rhythmicity until some time after their discharge from the hospital. We have been able affect this somewhat by the day-night lighting that we provide in the NICU but the premature baby still does not show the same circadian rhythm seen in full term babies. Therefore, we are looking at other possible contributing factors, one of which is melatonin, a hormone that helps all of us sleep. This hormone has been measured and found to be present in significant amounts in full term breast milk expressed at night but was not detected in milk expressed during the day; it has not been studied in preterm breast milk. This study will measure melatonin levels in breast milk, expressed at varying times of the day and night in mothers that have delivered prematurely between 24 weeks gestation and term. If the same results can be demonstrated, this may give us another way to help premature babies establish a day-night cycle by feeding them breast milk at the time of day it was expressed instead of feeding it randomly.