Circatidal rhythm of void volume in young healthy adults

Solar and lunar geophysical cycles are the major zeitgebers of biological oscillators, but most oscillating physiological systems are entrained to circadian rhythm. Although void volume oscillates, entrainment to geophysical cycle has not been shown. Study was done to determine if void volume oscillation of young healthy adults is entrained to geophysical cycle. Healthy undergraduates were placed on ad libitum regime of food and water intake, restricted exercise regime, but no intake of alcoholic beverages, caffeinated drinks, and tobacco. All voids were collected, timed, and measured for 72 h. Void volumes were corrected for body mass and binned to hourly time series. Hourly distribution of voids was tested for uniformity. Void volume time series was denoised, and the dominant oscillatory component was Hilbert transformed to analytical signal. The phase portrait was plotted in complex plane. The geometry of attractors in state space and recurrence were computed. There were 195 subjects, 128 males and 67 females. Geometrical mean age (range) was 26 years (18-37). Geometrical mean voids/day (50% inter-quantile range (IQR)) was 4 (4-5). Voids/hr were not uniformly distributed, . Geometrical mean void vol/kg/hr (50% IQR) was 0.8 ml (0.6-1.0). Void volume oscillation was rhythmic with peaks at 6.00 am and 6.00 pm, and troughs at 12.00 am and 12.00 pm, which coincided with Earth tides. The timescale of oscillation was 12 h. Phase portrait showed trajectories which converged on limit cycle attractor. Recurrence topology showed only periodic regime. Circatidal rhythm of void volume oscillation indicates entrainment to lunar geophysical cycle. The gravitation field of the moon is, therefore, the most likely zeitgeber.