Ambulatory blood pressure monitoring allows one to evaluate the blood pressure profile over a 24-hr period. It has been demonstrated that an average of daily pressure readings is more predictive of cardiovascular complications of hypertension than casual monitoring of blood pressure. On the other hand, there is no conclusive evidence that the higher degree of blood pressure variability found in hypertensive patients is related to increased hypertensive morbidity. The absence of a physiological decrease in arterial pressure during sleep is associated with complications of hypertension such as atherosclerosis and left ventricular hypertrophy as well as with impairments of the autonomic nervous system.