Biological 12-hour rhythm facilitates re-entrainment from circadian desynchrony and promotes psychological resilience during long-duration spaceflight

Abstract

Abstract Heart rate variability (HRV) reflects brain’s ‘vertical integration’ in association with functions of the default mode network (DMN) and salience network (SN). We investigate the role of 12-h components in HRV endpoints in facilitating adaptation to the space environment. Ambulatory 24-h electrocardiograms were obtained from 20 healthy astronauts (47.2 ± 5.9 years) before launch, twice in space (ISS01 on days 21.7 ± 2.9 and ISS02 on days 159.2 ± 46.1), and after return to Earth. Astronauts were classified depending on whether the 12-h amplitude of the TF-component of HRV during ISS01 versus pre-flight increased (Group I, N = 11) or not (Group II, N = 9). In Group I, the circadian acrophases of TF- and ULF-components were delayed during ISS01 versus pre-launch, whereas in Group II, the circadian acrophase of the MF1-band advanced. Lower HRV spectral power indicated psychological resilience during both ISS01 and ISS02 in association with the brain’s DMN and SN. Since adverse consequences of circadian desynchrony may be more pronounced due to phase advances than to phase delays, the differential behavior of the 12-h component between Groups I and II suggests its amenability to manipulation in interventions aimed at improving space adaptation.