Ambulatory blood pressure monitoring (ABPM) devices allow measuring changes of BP and heart rate (HR) by periodically inflating an arm cuff. Due to the low sampling rate, only simple indices of BP variability (V) have been considered so far, such as the standard deviation (SD) or the coefficient of variation (CV) of average BP levels over day and night periods. Our aim is to describe day-night changes in a specific feature of ABP complex dynamics: the fractal dimension (FD); and to evaluate whether FD provides complementary information to SD or CV in the description of 24h BPV.