Effect of shift work on blood pressure in middle aged workers

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We monitored blood pressure (BP) and heart rate (HR) every 15 minutes over 24 hours by a noninvasive ambulatory method in normotensive and hypertensive shift workers, once while on the day shift and once while on the night shift. The circadian variations in BP and HR were almost completely reversed with the switch from the day shift to the night shift in normotensive workers. In hypertensive workers, however, the circadian pattern was not completely reversed, i.e. BP and HR while on the night shift dropped to the lowest level of the day between 0000 and 0400. Thus, we found differences between normotensive and hypertensive workers in the effects of shift work and time of day on BP and HR.In a laboratory condition, we compared reactivity of blood pressure to three stress loading tests (mental arithmetic, cold face test, and head-up tilt) which were measured three times a day at 0900, 2100 and 0300. Blood pressure significantly declined during head-up tilt at 0300 in the hypertensive subjects, while their cardiac output showed no change during the test. We speculated that some mechanisms such as increasing the peripheral vascular resistance might be impaired in the hypertensive subjects during midnight, which is also related to the drop of blood pressure level between 0000 and 0400 in the hypertensive shift workers.