Definition of sleep state in the newborn infant by heart rate analysis.

The relationship of heart rate variability to sleep state was examined in 9 term newborn infants at one to three days of age. Spectral density plots of rhythmic variations in heart rate during active sleep and quiet sleep were characteristic and always distinct from each other. Heart rate and heart rate variability were significantly higher during active sleep than during quiet sleep. It was always possible to identify quiet sleep and active sleep during sleep periods by inspecting spectral density plots of the variations in heart rate. Similar relationships between heart rate variability and sleep states may exist in utero. Spectral analysis of heart rate variations may provide an indirect measurement of fetal sleep states, and such measurements might become useful in assessing fetal brain maturity and fetal well-being.