Cortical connectivity analysis for assessing the impact of microgravity and the efficacy of reactive sledge jumps countermeasure to NREM 2 sleep

Cognitive performance is crucial for long-term space missions, however, environments with zero gravity such as the International Space Station (ISS) or spacecrafts induce isolation and confinement. Many studies adapt the 6° head-down tilt bed rest (HDT) protocol in order to simulate the effects of weightlessness on sleep and psychophysiology in controlled environments. We investigated how weightlessness affects the sleep physiology and whether current countermeasures are efficient for sleep quality preservation by employing the head-down tilt (6°) bed rest protocol. We focused on NREM 2 sleep stage and we employed a 3-step analysis, which includes the estimation of the features that quantify the global functional organization of the cortical networks (mean cluster coefficient and characteristic path length), a local network analysis by estimating the connectivity strength of the various cortical regions with the thalamus and, quantifying changes in the functional importance (hub strength) for 22 cortical regions. The data were collected in the premises of the ENVIHAB Medical Research Facility of the Aerospace Centre in Cologne, Germany. The participants were 23 male individuals aged between 23 and 45 years old, assigned to either the control or experimental (sledge) group. Our design involved 60 days of HDT bed rest position (with the head tilted 6° downwards), with a two-week baseline and recovery period before and after the HDT duration. Our experimental design involved polysomnographic recordings (PSG) during sleep and we analysed the data collected 14 days before experiment onset (BDC-14) and during the 21st day of the HDT period (HDT21). Our results indicate that during N2 sleep in extreme environments, global networks characteristics and their feature connectivity are impaired. Degradation of N2 phase quality affects the declarative memory, especially for motor skills. The above finding highlights the importance of sleep quality regulation in astronauts in a way that enhances cognitive functions that is crucial in extreme environments.