Biorhythms of the nonpregnant human uterus.

Uterine activity was monitored continuously for periods of three or five days in two subjects. Biorhythms in the spontaneous variations of intrauterine pressure and frequency of contraction were demonstrated in both phases of the menstrual cycle. Statistically, the patterns conformed to 24-hour Fourier curves, with higher values synchronized by the periods of light. Positive statistical correlations were present between uterine activity and the circadian patterns of intrauterine temperature and norepinephrine concentrations in both urine and peripheral arterial plasma.