SummaryEighteen normal infants were studied in the first 2 wk of life during sleep and subsequently at their monthly birthdays for the first 4 months of life. The R-R interval was measured with an accuracy of 0.2 msec. Sleep staging was performed visually using electroencephalogram, electrooculogram, and electromyogram, and behavioral criteria. Our results show that the R-R interval and the beat-to-beat variability are, in general, smaller in rapid eye movement than in quiet sleep. The two sleep states, however, are best differentiated by the overall variability which is characteristically higher in rapid eye movement sleep. The R-R interval as well as the overall and the beat-to-beat variability show minimal values at 1 month and maximal rates of increase between 2 and 3 months of age, indicating that the R-R interval and R-R variability are not simple linear functions of age.SpeculationWe suspect that the large variability in the R-R interval in rapid eye movement sleep is attributable to the wide fluctuations in the activity of the sympathetic and parasympathetic nervous system characteristic of this sleep state. Because peripheral vasodilatation is known to occur in rapid eye movement sleep, we speculate further that the decrease in R-R interval in rapid eye movement sleep may be an adjustment to maintain the cardiac output and blood pressure.