Sleep structure and autonomic nervous system state during four months of isolation in a space analogue mission

Insufficient or disturbed sleep impairs nocturnal physiological recovery and may negatively affect autonomic nervous system (ANS) regulation. Astronauts are a particularly vulnerable group due to sustained workloads, isolation, and exposure to extreme operational conditions. The present study investigated the interaction between sleep structure and ANS state under different sleep conditions during a prolonged space analogue mission (SAM) conducted as part of the Scientific International Research in a Unique Terrestrial Station (SIRIUS-19) project.

Six healthy participants (three men and three women; age: 34.3 ± 5.7 years) were studied over eight nights during the four-month SIRIUS-19 mission: one night of undisturbed sleep pre-isolation, one night of undisturbed sleep post-isolation, and six nights during the isolation phase comprising three undisturbed nights, one night of complete sleep deprivation, and two nights of experimentally induced sleep fragmentation (repeated short awakenings vs. one prolonged awakening). Sleep structure and ANS state were assessed using a portable, self-applicable, medical-grade sleep recording system that captured electroencephalography (EEG), electrooculography (EOG), electrocardiogram (ECG), and plethysmography signals.

Significant differences in total sleep time were observed across different nights (p = 0.003). On nights of undisturbed isolation, participants achieved more than 7 h of sleep, while nights with sleep fragmentation was associated with reduced sleep efficiency (<80%). ANS state parameters differed significantly across conditions, including the pulse rate (PR) (p < 0.0001) and the heart rate variability (HRV) LF/HF ratio (p < 0.005), with the most pronounced autonomic activation occurring during the night of complete sleep deprivation.

Using a portable monitoring approach, this study demonstrates that nocturnal ANS regulation during prolonged isolation is relatively resilient to moderate sleep fragmentation, but is markedly affected by sustained sleep loss. These findings highlight the importance of preserving restorative sleep continuity when planning operationally demanding space missions and support the feasibility of portable sleep and ANS monitoring in extreme environments.