Circadian variations in blood pressure in junior medical staff were compared during routine working days and days receiving medical emergencies. The overnight work commitment of junior medical staff when receiving medical emergencies causes a loss in the normal circadian blood pressure rhythm, with overnight blood pressure falling 10/11 mmHg less than on routine working days. This supports the hypothesis that circadian blood-pressure changes are activity related rather than dependent upon hormonal variation. Long-term health effects of abolishing such normal circadian rhythms are not known.