NASA NTRS2024
Consequences of sleep and circadian rhythm disturbances include many neurophysiological sequelae, especially when combined with spaceflight stressors, and are therefore hypothesized to contribute to the development of optic disc edema observed in astronauts. Additionally, chronically elevated carbon dioxide (CO 2 ) levels on the International Space Station may contribute to the observed sleep disturbances in astronauts. Here we determined if the spaceflight analogue head-down tilt bed rest (HDTBR) combined with elevated CO 2 levels influenced sleep and core temperature and was associated the development of optic disc edema. Healthy participants (5 female, 6 male) aged 25-50 years, underwent thirty days of strict 6° HDTBR in a mild hypercapnic environment (ambient PCO 2 = 4 mmHg) with measures of sleep, 24-hour core temperature, overnight arterialized CO2, and Frisén grade edema collected pre-HDTBR, on HDTBR days 4, 17, 28, and post-HDTBR days 4 and 10. During all HDTBR time points, sleep, core temperature, and overnight arterialized CO 2 were not statistically significantly different than the pre-HDTBR measurements. However, independent of the HDTBR intervention, the odds ratios (mean [95% CI]) for developing Frisén grade optic disc edema were statistically significant for each hour below the mean total sleep time (2.2 [1.1-4.4]) and stage 2 NREM sleep (4.8 [1.3-18.6]), and above the mean for wake after sleep onset (3.6 [1.2-10.6]) and for each 0.1°C decrease in core temperature amplitude below the mean (4.0 [1.4-11.7]).These data suggest that optic disc edema occurring during HDTBR was more likely to occur in those with short sleep duration and blunted temperature amplitude.