The increased rate of heart disease and accidents associated with shift work could be because the body's circadian rhythms are programmed to slow down at night and increase during the day, regardless of whether the body is active or not.
In a study published in the 17 October Circulation, scientists at the University of Milan studied 22 male steel workers, who worked three types of shift, rotated weekly. The shifts included 06:00–14:00, 14:00–22:00 and 22:00–06:00. For each shift the workers were given two days to adjust, and then ECG recordings were monitored continuously over 24 hours. The researchers also conducted spectral analysis of heart rate variability, which provided normalized markers of sympathetic (LFnu) and vagal (HFnu) modulation of the sinoatrial node activity and of the sympathovagal balance (LF/HF). Dr Raffaello Furlan and his team found that regardless of work schedule, the sympathetic and vagal autonomic controls maintained the same pattern: rising during the day and subsiding during the night. 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 LF nu 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 the morning and evening.
Dr Furlan commented "This resistance of the body's internal clock to change with varied work schedules indicates that people don't adapt as easily as we think to shift work, and could explain why shift workers are at higher risk." The team also found that the circadian oscillations of body temperature and plasma cortisol compensated slightly, but not enough to be at a maximum during the working period. Dr Furlan postulated that the rigidity of sympathetic and vagal autonomic control over a 24-hour period places a strain on the heart of shift workers, and could play a part in the excessive rate of cardiovascular diseases among this group. In addition, the reduced values of temperature and plasma cortisol while working could contribute to sleepiness or diminished alertness, which may facilitate errors and accidents.