Patients with essential hypertension have disturbed autonomic cardiovascular regulation and circadian pacemaker function. Recently, the biological clock was shown to be involved in autonomic cardiovascular regulation. Dutch scientists therefore wanted to determine whether or not enhancement of the functioning of the biological clock by repeated night-time melatonin intake might reduce ambulatory blood pressure in patients with essential hypertension. They conducted a randomised, double-blind, placebo-controlled, crossover trial in 16 men with untreated essential hypertension to investigate the influence of acute (single) and repeated (daily for 3 weeks) oral melatonin (2.5 mg) intake 1 h before sleep on 24-h ambulatory blood pressure and actigraphic estimates of sleep quality. Repeated melatonin intake reduced systolic and diastolic blood pressure during sleep by 6 and 4 mm Hg, respectively. The treatment did not affect heart rate. The day–night amplitudes of the rhythms in systolic and diastolic blood pressures were increased by 15 and 25%, respectively. A single dose of melatonin had no effect on blood pressure. Repeated (but not acute) melatonin also improved sleep. Improvements in blood pressure and sleep were statistically unrelated.