Beyond Microgravity: Circadian Desynchronization, Cardiovascular Remodeling, and Physiological Resilience in Long-Duration Human Spaceflight

Long-duration spaceflight exposes astronauts to environmental and physiological conditions that challenge circadian synchronization, sleep regulation, and cardiovascular homeostasis. Understanding the interactions among these processes is essential for preserving physiological performance and operational safety during future lunar and interplanetary missions. This narrative review aimed to examine the relationship between circadian disruption, sleep disturbances, and cardiovascular adaptation during prolonged exposure to microgravity, while evaluating the available evidence concerning preventive countermeasures. Scientific literature addressing aerospace physiology, chronobiology, sleep medicine, and cardiovascular regulation was critically synthesized, with particular emphasis on observational studies involving astronauts, physiological investigations, and specialized reviews. The findings revealed persistent sleep restriction during orbital missions, with reported mean sleep durations of approximately six hours per night. Changes in cardiovascular autonomic regulation were documented through alterations in heart rate variability, including modifications in fractal scaling and spectral power. Hemodynamic investigations demonstrated reductions in arterial pressure and systemic vascular resistance, accompanied by increases in cardiac output and stroke volume during prolonged microgravity exposure. Although these findings indicate substantial physiological adaptation, the independent contribution of circadian misalignment to cardiovascular deconditioning remains uncertain. Countermeasures involving controlled lighting, structured sleep schedules, exercise training, and mechanical loading address different physiological demands, but their combined effectiveness has not been adequately established. The available evidence is limited by small astronaut populations, methodological heterogeneity, and insufficient simultaneous monitoring of circadian and cardiovascular parameters. An integrated approach combining biological rhythm assessment, cardiovascular monitoring, and individualized preventive interventions may improve the characterization of physiological adaptation during extended missions. Further longitudinal investigations are required to clarify causal mechanisms and establish effective strategies for maintaining astronaut health during sustained human exploration beyond Earth.