Twenty-four-hour blood pressure is not dependent on endogenous circadian rhythm

The effects of shifted working and sleeping phases on the diurnal blood pressure rhythm were investigated in 15 physically working industrial shift workers at a slowly rotated three-shift system. Ambulatory 24-h blood pressure monitoring was performed during the morning and night shifts. In the two shifts the mean 24-h blood pressure was identical. There were no differences in the blood pressure levels in the sleeping phases or in the working periods between the two 24-h cycles. Diurnal blood pressure fluctuations had equal amplitudes. Corresponding to the lag between the working period there was a phase difference of 8 h between the 24-h blood pressure curves. At this lag, there was a high correlation between the mean hourly blood pressure values (r = 0.683). Twenty-four-hour blood pressure curves during the first and last day of a night shift were nearly equal. Thus the effects of shift rotation on the 24-h blood pressure profile were fully expressed within the first 24 hours. The immediate and complete adaptation of the 24-h blood pressure curve to shifted activity and sleeping phases indicates that activity determines the diurnal blood pressure profile. The blood pressure is largely independent of internal circadian rhythm.