Background: Spaceflight causes changes in the cardiovascular control system. The aim of this study was to evaluate postflight recovery of linear and nonlinear neural markers of heart rate modulation, with a special focus on day-night variations.
Material/Methods: Twenty-four-hour Holter ECG recordings were obtained in 8 astronauts participating in space missions aboard the International Space Station (ISS). Data recording was performed 1 month before launch, and 5 and 30 days after return to Earth from short- and long-term flights. Cardiovascular control was inferred from linear and nonlinear heart rate variability (HRV) parameters, separately during 2-hour day and 2-hour night recordings.
Results: No remarkable differences were found in the postflight recovery between astronauts from short- and long-duration spaceflights. Five days after return to Earth, vagal modulation was significantly decreased compared to the preflight condition (day: p=0.001; night: p=0.019), while the sympathovagal balance was strongly increased, but only at night (p=0.017). A few nonlinear parameters were reduced early postflight compared to preflight values, but these were not always statistically significant. No significant differences remained after 30 days of postflight recovery.
Conclusions: Our results show that 5 days after return from both short- and long-duration space missions, neural mechanisms of heart rate regulation are still disturbed. After 1 month, autonomic control of heart rate recovered almost completely.

Keywords: Nonlinear Dynamics, heart rate variability, autonomic regulation, Microgravitational adaptations