Variability in the period of the blood pressure circadian rhythm in human beings.

In studies and assessments of human beings done in natural settings, it is assumed that the period tau of circadian rhythms, including ones of systolic (SBP) and diastolic (DBP) blood pressure, is equal to 24 hours. To test this hypothesis, SBP and DBP rhythms were studied in 112 medication-free, non-hospitalized subjects (62 males, 47.1 + 2.0 years [x +/- SEM], and 50 females, 54.5 +/- 2.1 years) by 48 h ambulatory blood pressure monitoring (ABPM). Of these, 26 were hypertensive (diurnal SBP > 140 mmHg and diurnal DBP > 90 mmHg) and 86 normotensive. All subjects were synchronized by their habitual daytime activities from approximately 08:00 h to approximately 23:00 h +/- 1 h and by sleep at night. The BP was assessed at 15-minute intervals during a continuous 48h span using a Spacelabs model #90207 ABPM. The time series data of each subject were individually evaluated by power spectra analysis for the prominent tau of the SBP and DBP rhythms. The prominent tau differed from 24 hours in 22/112 subjects for SBP and in 16/112 subjects for DBP. Generally, in these individuals the tau was less than 24 hours. The occurrence of non-24 h tau's was more frequent in hypertensive than normotensive subjects; the difference between the groups in the distribution of the prominent tau's by class (tau = 24 h, tau = 12, 12 h > tau < 24 h, etc.) was statistically significant (chi 2 test = 19.1; p < 0.001). No difference in the distribution of tau's of blood pressure was detected according to the subject's age and gender. These findings suggest that ABPM done only for a duration of 24 h may be too short to characterize accurately the features of the day-night variation in human BP, including the precise period of its rhythm.