The time (chronos) structure of life (bios) is the topic of a computer-implemented science (logos), chronobiology. Chronobiologic methods for resolving physiologic time structure provide a more reliable mean, the MESOR, and other parametric dynamic measures of blood pressure variability. The amplitude is a measure of predictable extent of change and the acrophase a measure of timing of overall high values within each cycle of the rhythmic function. Such chronobiologic endpoints are often more sensitive than the mean for separating groups at different risk of developing high blood pressure. About 24-h (circadian) rhythms account for a sizeable part of predictable variability in blood pressure. Methodology to assess the characteristics of circadian blood pressure rhythms also provides time-varying reference limits for the interpretation of single measurements. Deviant blood pressures can be quantified by reference to such circadian-stage-dependent limits, derived from the automatically monitored blood pressure profiles of healthy peer groups. Excess or deficit in blood pressure can also be assessed as a hypertensive or hypotensive index integrated over 24 h. Chronobiologic monitoring has wide uses in practice as well as research, whether it is carried out by self- or automatic measurements, once the data are analyzed by appropriate computer methods that are now readily available.