What time is the "biologic zero hour" of circadian variability?

Most ambulatory blood pressure monitoring (ABPM) studies have used a mechanical clock as the reference time, but there is no biologic background for assuming that midnight by the mechanical clock is zero hour by the biologic clock. The aim of this study was to determine the biologic zero hour as the zero reference time by evaluating the circadian rhythm of blood pressure, heart rate, and activity. Twenty healthy medical students (18 men, 2 women, mean age 26 years old) were recruited and blood pressure, heart rate, and physical activity were monitored simultaneously by an ABPM device every 30 min for 48 h. Four concepts of zero time were selected in this study and analyzed regarding biologic zero hour: 24:00 by the mechanical clock (clock time); the time of awakening, based on a diary (diary time); the time of a sudden increment in physical activity in the morning (activity time); and the middle of the total sleeping time, based on the diary (midsleeping time). The awakening time is a better individual index than the mechanical clock, and the midsleeping time as the zero reference point is better than the awakening time. We assessed the reproducibility of the data regarding the circadian troughs between the first and second day. The reproducibility of the day-to-day variation of the blood pressure and heart rate was poor. The reproducibility of physical activity was fairly good, but the magnitude of activity was small. A 48-h monitoring profile is superior to a 24-h monitoring period.