Deep space exploration: Sleep and circadian rhythms in heart and brain function.

During deep space exploration, astronauts are exposed to external environmental stressors, including microgravity and ionizing radiation. They also experience internal physiological disruptions, such as circadian rhythm dysregulation and sleep disturbance. Within this complex stress environment, functional alterations may occur in multiple organ systems, particularly the heart and brain. This review examines alterations in heart and brain function during spaceflight and explores their potential relationships with microgravity, circadian rhythm dysregulation, and sleep disturbance. Evidence from spaceflight observations, spaceflight experiments, and ground-based analog or mechanistic studies suggests that these changes may involve mitochondrial dysfunction, oxidative stress, metabolic disturbance, autonomic dysregulation, and inflammatory responses. However, direct causal evidence in astronauts remains limited, and many mechanistic findings still require validation under actual spaceflight conditions. By clarifying how circadian rhythm dysregulation and sleep disturbance may be linked to alterations in heart and brain function in the spaceflight environment, this review aims to inform monitoring and countermeasure strategies for forthcoming lunar and Mars missions.