Blood pressure (BP) varies throughout the day owing to interactions between the sympathetic nervous and the renin-angiotensin systems. When awake BP is controlled by sympathetic nervous system activity but during sleep the renin-angiotensin system becomes more important. The lower BP during sleep is a more powerful predictor of outcome than the awake BP. Certain individuals do not have the fall in BP with sleep and this worsens the outcome. Inadequate handling of sodium by the kidney is an important factor preventing this BP fall. The different drug classes have varying effects on BP during 24 h. Drugs that act independently of the two controlling systems have a similar effect at day and night. Drugs that act on the sympathetic nervous system have a greater effect during the day and little effect during sleep unless the sympathetic system is still active. Drugs that act via the renin-angiotensin system have a greater effect during sleep. Controlling BP during sleep may improve outcome.