Our purpose was to evaluate the benefits of cycled light (CL) versus near darkness (ND) on health in preterm infants born at <31 weeks' gestational age.Randomized, interventional study comparing infants receiving (1) CL from birth, (2) CL at 32 weeks' postconceptional age (PCA), and (3) CL at 36 weeks' PCA in transition for discharge home. Statistical significance was assessed with segmented mixed general linear models, analysis of covariance, general estimating equations, chi(2), and Fisher's exact procedure.Infants receiving CL at birth and 32 weeks' PCA gained weight faster than infants not receiving CL until 36 weeks' PCA. There were no differences among the groups in length of hospitalization stay or number of ventilator days, but the power was low for these variables.These findings suggest that CL has significant weight gain benefits over ND, and there are no short-term advantages of ND over cycled light for health in preterm infants.