The developmental sequence of sleep cycles has been found to be a useful index of central nervous system maturation in premature infants. To determine the effects of severe reversible stress on the maturation of sleep cycles, 6- to 8-hour sleep studies were done on 10 premature infants with severe hyaline membrane disease (HMD) and 10 healthy premature babies. The studies were done in the neonatal intensive care unit and included patients from 30 weeks' gestation to term and in all stages of the disease. Prior studies were confirmed showing that percentage of quiet sleep increases, transitional sleep changes little, and active sleep decreases from 30 weeks' gestation to term. In infants with severe HMD, the percentage of quiet sleep was less and active sleep higher than in unstressed infants of similar age who acted as controls. Once the infants recovered from HMD, sleep patterns became similar to those found in the control group. These data suggest that when respiratory distress occurs in the premature infant, there is a transient delay in central nervous system maturation as measured by sleep cycle analysis.