Newborn jaundice (yellow discoloration of skin) is a universal problem in preterm infants. Jaundice results from elevated bilirubin levels in blood and can lead to brain damage and long-term disabilities in severe cases. The treatment for jaundice is phototherapy (PT). There are well established guidelines for starting and stopping PT in full-term infants. However, there is a lack of evidence to guide similar decisions in preterm infants at different gestational ages and birth weights. In contrast to its potential negative impact, bilirubin is also known to be a helpful antioxidant and has been shown to protect against some diseases such as heart disease and cancer in adults. Whether bilirubin at low levels may have health benefits in preterm infants is not known. This translational study will use an animal model of newborn jaundice and a clinical pilot study in preterm human infants to discover biomarkers to know when bilirubin is either helpful or harmful, thus helping medical providers make evidence-based clinical decisions about treatment of jaundice in preterm newborns.