Thirty-five normal two-month-old infants had nighttime followed by daytime polygraphic recordings. Heart rates were calculated every minute in active and quiet sleep states. A difference in mean heart rates was found between the two states and between the two recordings. Rates were lower at night than during the day (P < 0.0001), regardless of the sleep state. During nighttime or daytime recordings as a function of sleep cycles or during sustained sleep episodes, heart rates were minimal in the middle of recordings, but differences were statistically significant for only a few results (mainly in QS). Intrasleep awakening led to a marked increase in heart rate after sleep was resumed, although differences were only statistically significant when the awakened infant was fed. Infants with episodes of periodic breathing had lower mean heart rates throughout the recordings, but differences were not statistically significant. Respiratory and heart rates showed similar changes during the recordings, and a statistically significant correlation was found between the two measurements.