Daytime and nighttime ambulatory blood pressures should be calculated over the true sleep/waking cycle and not over arbitrary periods.

The present study was done to compare the values of mean daytime and nighttime blood pressure (BP) calculated over arbitrary periods to those calculated over the true retiring and rising times of the individual patients. A total of 88 individuals, including 55 untreated hypertensives (office BP > 140/90 mm Hg) and 33 normotensives, were recruited. Ambulatory BP was monitored over 24 h during the normal routine of the patient. The patient was requested to trigger a recording on going to bed and on rising in the morning to clearly identify these periods. The mean daytime and nighttime values were calculated over arbitrarily defined periods (6 AM to 10PM daytime and 10 PM to 6 AM nighttime) and as a function of the true retiring and rising times of the individual patients. The true daytime BP was significantly higher than the value calculated over the arbitrary period and the true nighttime BP was significantly lower than the value calculated between 10 PM and 6 AM (paired t test, P < .05). Employing a cutoff value of 135/85 mm Hg for daytime BP, a significant proportion of patients classified as normotensive when daytime BP was calculated over arbitrary periods were in fact classified as hypertensive from the values calculated over true retiring and rising times.