Circadian clocks in all tissues confer temporal organization to the physiology and behavior of organisms. Rhythms of the cardiovascular system have been scrutinized because of the morning peak of adverse cardiovascular events and because night and rotating shift work have been associated with heart disease and biomarkers of elevated cardiometabolic risk. Animal models support the important role that the clock plays in the heart. External disruptions such as jetlag and internal disruptions such as loss of clock function contribute to poor heart health. In this chapter, we review key findings from animal models of circadian disruption and from experiments in humans designed to isolate the effects of the circadian clock on cardiovascular physiology.

Daily and seasonal rhythms are programmed by neural circuits that anticipate predictable changes in the environment (i.e., temperature, food, predation). The time and duration of daily light exposure is a strategic cue used to predict changes in the environment that determine fitness and survival. Light is transduced by a specialized visual system that serves as an irradiance detector. These inputs are processed and encoded by the suprachiasmatic nucleus (SCN), which serves as the body’s daily clock and annual calendar. The SCN encodes time-of-day and photoperiod to regulate downstream systems via multiple routes (e.g., melatonin, cortisol, feeding, body temperature). A deeper understanding of SCN timekeeping circuits, photoperiodic encoding mechanisms, and light-driven cellular adaptations is imperative for understanding plasticity and pathology in multiple biological systems.