 Apnea of prematurity is a common morbidity among premature infants. Recent studies suggest an association between apnea and neurodevelopmental impairment. Our objectives are to determine normative values of oxygen saturation in late preterm infants -reference group. In infants born 28-35 weeks requiring caffeine- study group and infants born 28-35 weeks not requiring caffeine-control group we will be evaluating the frequency of hypoxic events. Recruitment time: February 2015 until December 2016. 