Circadian Biology and Its Importance to Intensive Care Unit Care and Outcomes

Circadian rhythms are an integral part of life on earth. Circadian rhythms play a fundamental role in homeostasis as they ensure coordination between the environment and an organism's behavior and physiology. This coordination is called entrainment. Entrainment depends on environmental cues known as zeitgebers. Human zeitgebers include light (primary zeitgeber), sleep, eating, exercise, and activity. Circadian rhythms are disrupted in critically-ill patients due to both critical illness and current intensive care unit (ICU) practices. Disruptions in circadian rhythms are tightly linked with ICU sleep disruption. Together these entities potentiate numerous adverse outcomes including delirium, metabolic derangements, cardiovascular instability, and immune compromise. Herein, we will highlight potential areas for care improvement via chronobundles. We suggest bright light during the day, maintaining darkness, and protecting sleep at night, intermittent rather than continuous feeds, and activity via mobilization during the day. Optimizing circadian rhythms is a low-risk intervention that is underutilized in current ICU practice. This optimization could be a powerful tool in helping to improve outcomes in the critically-ill patient.