What is the 'normal' 24 h, awake, and asleep blood pressure?

The clinical use of ambulatory blood pressure monitoring requires that a 'normal' limit be identified. However, because the relationship between blood pressure and risk is continuous, any such limit is arbitrary. Various different methods have been used to define a normal limit, and can be divided into those that rely solely on blood pressure measurement, and those that relate blood pressure to target-organ damage or risk. First, several population studies of 'normotensive' people have identified the 95th centile of ambulatory blood pressure, with generally consistent findings, but with the methodological flaw that 'hypertensive' individuals must first be excluded. Second, the ambulatory blood pressure that corresponds to a threshold value of clinic blood pressure (e.g. 140/90 mmHg) can be identified from the regression equation linking the two measures of blood pressure. Third, the percentage of readings greater than a threshold value (the 'blood pressure load') can be specified; this method is arbitrary, and makes the unwarranted assumption that there is a threshold value above which blood pressure causes harm. Fourth, the ambulatory blood pressure above which the prevalence of target-organ damage begins to increase significantly can be identified. Fifth, the relationship between ambulatory blood pressure and cardiovascular risk can be examined in prospective studies. Despite these very different approaches, with a few notable exceptions there is reasonable agreement between the methods, the consensus being that, for daytime ambulatory blood pressure, the normal value is about 135/85 mmHg.