Modifications of cardiac autonomic profile associated with a shift schedule of work

Background—Shift work is associated with an increased rate of cardiovascular diseases and accidents. Discordance between circadian rhythms of stress-related biological variables and the work-sleep schedule might explain the reduced efficiency of work. It is not clear whether a shift schedule of work may induce similar discordance in the 24-hour oscillatory pattern of the cardiac autonomic control in respect to the work-sleep periods. Methods and Results—Twenty-two healthy male blue-collar shift workers underwent 24-hour ECG recordings during each of the 3 different shifts (first, 6 AM to 2 PM; second, 2 to 10 PM; third, 10 PM to 6 AM). Spectral analysis of heart rate variability over 24 hours provided the normalized markers of cardiac sympathetic (LFnu) and vagal (HFnu) modulation of the sinoatrial node activity and of the sympathovagal balance (LF/HF). LFnu and LF/HF exhibited 24-hour oscillations with different times of maximum and minimum in accordance with the working and sleeping periods, respectively. Lower values of LFnu and LF/HF suggestive of a reduced cardiac sympathetic modulation were present when the job task was performed at night compared with the values observed when the work was performed during morning and evening. Conclusions—Continuous weekly changes of time of maximum and minimum in the cardiac sympathetic and vagal autonomic control may play a role in the excessive rate of cardiovascular diseases in shift workers. The reduced values of the indexes of cardiac sympathetic modulation during night work might be related to the presence of sleepiness or diminished alertness, which in turn could facilitate errors and accidents.