Blood pressure reduction at night: sleep and beyond.

Blood pressure fall at night is commonly used to classify subjects as 'dippers' or 'non-dippers'. Such a classification is poorly reproducible, however, due to interference by a number of confounders. These include methodological problems with ambulatory blood pressure monitoring at night due to a variable hydrostatic difference between the arm cuff and the heart. Even correcting for this variable, however, does not significantly improve the reproducibility of the nocturnal blood pressure fall, which probably depends to a large extent on other factors, such as the level of daytime activity and differences in sleep patterns.