B C Kalmath

Blood pressure is a biological variable that varies with time. A series of factors linked to daily activities and independent neurohormonal mechanisms cause blood pressure to vary significantly within the 24 hours of the day: it increases during physical activity and decreases considerably following acute exercise (post-exercise hypotension); it increases in conditions of physical or emotional distress and tends to decrease after meals and during sleep.1 Blood pressure variation throughout the sleep-wake cycle is well known, with lower values observed during night time sleep than in daytime wakefulness. The advent of non-invasive methods to measure blood pressure for 24 hours such as ambulatory blood pressure monitoring (ABPM) shed more light onto the behaviour of blood pressure. Changes in blood pressure during sleep have also been associated with increased risk of cardiovascular and renal disease in the long term: it is the classical description of inadequate or absent drops in blood pressure during sleep seen in attenuated dippers and non-dippers, respectively, associated with left ventricular hypertrophy, hypertensive retinopathy, and proteinuria.2 A mathematical formula based on the different blood pressure levels observed in subperiods of the sleep-wake cycle was recently proposed to estimate the early morning rise in blood pressure. Pronounced rises in blood pressure during this time of the day (“early morning surge”) have also been used to independently predict mean 24-hour, wake, and sleep blood pressure and the Blood pressure variability3