Shifts in the light-dark cycle increase unpredictability of the cardiovascular system

Physiological variables such as heart rate (HR) and blood pressure (BP) exhibit long-term circadian rhythms, which can be disturbed by shift work. On the other hand, short-term oscillations in HR and BP have a high prognostic value. Therefore, we aimed to determine if the short-term variability, complexity and entropy of HR and BP would be affected by a regular light/dark (LD) cycle and phase delay shifts of the LD cycle, leading to chronodisruption. Telemetry-monitored rats were exposed first to the regular LD cycle and then to shifts in LD for 8 weeks. On the basis of long-term HR and BP recording and evaluation, we found circadian rhythms in HR and BP variability, complexity and entropy under regular LD cycles. Short-term exposure to shifts disturbed circadian rhythms of HR and BP variability, complexity and entropy, indicating chronodisruption. The power of circadian rhythms was suppressed after 8 weeks of phase delay shifts. Long-term exposure to shifts increased variability (p = 0.007), complexity (p < 0.001) and dark-time entropy (p = 0.006) of HR but not BP. This is the first study demonstrating long-term recording and estimation of HR and BP variability, complexity and entropy in conscious rats exposed to irregular lighting conditions. After long-term phase delay shifts, short-term variability of HR was less predictable than in controls. This study suggests that changes in short-term HR and BP oscillations induced by long-term shift work can negatively affect cardiovascular health.