Diurnal rhythm of fetal behavioral state.

To test for the presence in utero of diurnal periodicity in the amount of time spent in organized behavioral states, seven records of 24-hour duration were obtained from each of five chronically instrumented fetal baboons between 144 and 158 days gestation (term = 175 days). Concordance of fetal breathing activity, heart-period variability, and electroencephalographic parameters were used to define two distinctive patterns of fetal physiological activities. One pattern was characterized as EEG activity dominated by tracé alternant, reduced heart-period variability, and fewer breaths in epochs of fetal breathing. This fetal behavioral state (1FB) is analogous to quiet sleep in infants. A second pattern was characterized by the relative absence of tracé alternant, increased heart-period variability, and fetal breathing activity. This fetal behavioral state (2FB) is analogous to active sleep. Cycles of these states were present 29% of time, with a duration of approximately 26 minutes and a 1.7:1 predominance of 2FB/1FB. Cosinor analysis across fetuses revealed a significant (p < .01) 24-hour periodicity of the time spent by the fetus in organized behavioral states, with a peak around 1400 (lights-on 0700 to 1900) and a peak-nadir fluctuation of 15%. These periodicities in the incidence of organized state were significant (p < .01) in three fetuses, and approached significance (< .09) in the two others. Data demonstrate a diurnal rhythmicity in fetal behavioral states, with less time spent in organized state at night than during the day.