Repeated long-duration space missions facilitate brain plasticity at night and psychological resilience during daytime

Repeated long-duration spaceflights of a healthy astronaut, 4 years apart, facilitated microgravity-induced brain-plasticity and psychological-resilience, observed about 20 days after launch (ISS01) during the second mission. Sleep duration lengthened, sleep quality improved, and spectral power of heart rate variability indices (HRV, msec 2 ), co-varying with activity of the salience network (SN), increased at night: HF-component (0.15–0.50 Hz) from 345.1 to 557.5 (P = 0.0580), and HF-band (0.30–0.40 Hz) from 52.7 to 101.5 (P = 0.0417). Spectral power of HRV indices during daytime, which correlate negatively with psychological-resilience, decreased: HF-component from 321.0 to 249.8 (P = 0.0240), and HF-band from 58.5 to 36.7 (P = 0.00002). LF-component and LF-band, reflecting activity of the default mode network, did not change significantly. Contrary to the first mission, no significant changes in 24-hour acrophases of HRV endpoints were observed about 20 days after launch during the second mission, but the 12-hour acrophase of TF-HRV underwent a remarkable phase delay from − 216° to -52° (P < 0.0001). The 12-hour component could thus consolidate the circadian system and contribute to a better adaptation in space by taking advantage of brain plasticity at night and psychological resilience during daytime.