The organization of sleep activity in stages of different depth is reflected by consistent changes in blood pressure that account for the major part of the day-night blood pressure difference. On the other hand, different mechanisms may underlie dysregulation of nocturnal blood pressure. Cyclic variations in autonomic nervous system activity play an important role in the mediation of the influences of sleep and wakefulness on blood pressure. In addition, several physiologic substances that are knoiwn to induce sleep or arousal are knoiwn to exert actions on blood pressure. Hence, derangements in autonomic nervous system activity, either primitive or secondary to alterations in the circadian rhythm of a variety of neurohumoral factors, are reflected in changes of the circadian blood pressure profile. Important additional influences of sleep on blood pressure may be exerted through respiratory variations, so that sleep-disordered breathing is bound to alter nocturnal blood pressure. Finally, insomnia has to be taken into account as a major cause of sleep-related alteration of the circadian blood pressure profile. The number of medical disorders that can cause insomnia is huge, and includes many if not all of the conditions in which a loss or reversal of the physiologic blood pressure fall at night is found to be prevalent. Unfortunately, objective sleep studies have been performed only in studies of a minority of these disorders, and further studies to assess the pathophysiologic mechanisms actually involved in causing sleep disturbances in each pathologic condition are necessary.