1. Blood pressure, heart rate, plasma noradrenaline and electroencephalographic variables were measured simultaneously during sleep in five healthy males before and after slow-release clonidine (300 μg orally) or placebo.
2. The blood pressure fall during sleep correlated significantly with the depth of sleep.
3. Clonidine significantly reduced systolic blood pressure during sleep and profoundly reduced total paradoxical sleep duration.
4. Plasma noradrenaline declined progressively during the sleep period and concentrations were significantly lower after clonidine compared with placebo.