WHAT TIME IS THE BIOLOGIC ZERO HOUR OF CIRCADIAN VARIABILITY

Most ambulatory blood pressure monitoring (ABPM) studies have used a m echanical clock as the reference time, but there is no biologic backgr ound for assuming that midnight by the mechanical clock is zero hour b y the biologic clock. The aim of this study was to determine the biolo gic zero hour as the zero reference time by evaluating the circadian r hythm of blood pressure, heart rate, and activity. Twenty healthy medi cal students (18 men, 2 women, mean age 26 years old) were recruited a nd blood pressure, heart rate, and physical activity were monitored si multaneously 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 aw akening, based on a diary (diary time); the time of a sudden increment in physical activity in the morning (activity time); and the middle o f 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 t he awakening time. We assessed the reproducibility of the data regardi ng the circadian troughs between the first and second day. The reprodu cibility of the day-today variation of the blood pressure and heart ra te 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. (C) 1997 American Journal of Hy pertension, Ltd.