Investigators conducting hypertension trials with ambulatory blood pressure (BP) monitoring have been analyzing study results using arbitrary times for day (wakefulness) and night (sleep). We prospectively evaluated the impact of using arbitrary times instead of patient reported awake and sleep times on mean 24-h, awake, and sleep BP, BP loads, and the awake-sleep BP difference in 50 subjects. Daytimes and nighttimes were derived from popular, arbitrary times reported in the literature. Compared to actual awake and sleep periods, arbitrary day and night division caused no significant differences in the mean awake and sleep BPs. However, limits of agreement for BP values derived for the actual and arbitrary times of wakefulness and sleep were substantial especially during sleep (awake systolic BP, -4 to 7 mm Hg; awake diastolic BP, -2 to 4 mm Hg; sleep systolic BP, -12 to 7 mm Hg; and sleep diastolic BP -7 to 4 mm Hg). Sleep BP loads (proportion of BPs > 120/80 mm Hg) were altered by greater than 10% in 20% to 30% of the subjects, depending on choice of time schedule. These data demonstrate that the calculation of BP and BP load during sleep may be altered by use of arbitrary, rather than actual, times of wakefulness and sleep in 24-h studies of ambulatory BP.