Mars 520-d mission simulation reveals protracted crew hypokinesis and alterations of sleep duration and timing

Abstract

The success of interplanetary human spaceflight will depend on many factors, including the behavioral activity levels, sleep, and circadian timing of crews exposed to prolonged microgravity and confinement. To address the effects of the latter, we used a high-fidelity ground simulation of a Mars mission to objectively track sleep-wake dynamics in a multinational crew of six during 520 d of confined isolation. Measurements included continuous recordings of wrist actigraphy and light exposure (4.396 million min) and weekly computer-based neurobehavioral assessments (n = 888) to identify changes in the crew's activity levels, sleep quantity and quality, sleep-wake periodicity, vigilance performance, and workload throughout the record-long 17 mo of mission confinement. Actigraphy revealed that crew sedentariness increased across the mission as evident in decreased waking movement (i.e., hypokinesis) and increased sleep and rest times. Light exposure decreased during the mission. The majority of crewmembers also experienced one or more disturbances of sleep quality, vigilance deficits, or altered sleep-wake periodicity and timing, suggesting inadequate circadian entrainment. The results point to the need to identify markers of differential vulnerability to hypokinesis and sleep-wake changes during the prolonged isolation of exploration spaceflight and the need to ensure maintenance of circadian entrainment, sleep quantity and quality, and optimal activity levels during exploration missions. Therefore, successful adaptation to such missions will require crew to transit in spacecraft and live in surface habitats that instantiate aspects of Earth's geophysical signals (appropriately timed light exposure, food intake, exercise) required for temporal organization and maintenance of human behavior.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Activity profiles of crewmembers measured by continuously worn wrist actigraphs throughout the 520-d simulated mission. (A) The crew’s daily mean (gray points) time (hours) spent in active wakefulness (red trend line), sleeping (blue trend line), and resting (green trend line) across the mission (for information on statistical analyses, see SI Appendix, SI Text). (B–D) The crew’s mean (SE) time in each behavioral state for each consecutive 130-d MQ (red arrow shows simulated midmission landing on Mars). There was a systematic decrease across MQs in active wakefulness (B) and systematic increases in both sleep time (C) and waking rest time (D). F test P values for these effects are shown in each graph; post hoc tests between MQs: *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. (E) The crew’s daily mean (gray points) intensity of activity (counts per min) across the mission during active wakefulness (red trend line), sleep (blue trend line), and rest (green trend line). (F–H) The crew’s mean (SE) intensity of activity in each state by MQ. Activity levels declined in MQ 2 and much of MQ 4 relative to the first MQ, but rose in the final 20 d of the last MQ (F test P values are shown in F–H). The sharp increases in active wake time (A) and intensity (E) in the final 20 mission days, and the commensurate sharp decreases in sleep and rest times (A), were significantly different from mean values for these variables relative to two sequential 60-d periods immediately before the final 20 d of the mission (P < 0.002).

Fig. 2.

Cumulative functions over 520 d of mission confinement for each crewmember’s waking activity levels (A), time spent in sleep (B) and rest (C), and PVT-B error rate (D). Examination of data from crewmembers d and f illustrate the interindividual differences among the crew in reaction to the prolonged mission confinement.

Fig. 3.

Double raster plots of sleep (black) and wake (white) and spectral plots (blue and yellow) from actigraphically derived sleep and waking throughout the 520-d mission for crewmembers a, b, c, d, e, and f (A–F, respectively). Rest was classified as wake for these analyses. Spectral analyses to evaluate sleep–wake periodicity were performed on 1-min actigraphic epochs based on the power spectral density by using the periodogram method (16), multiplying the data with a 90-d rectangular window and taking the squared magnitude of the discrete time Fourier transform. The peak frequency was estimated by a 3-point quadratic interpolation based on the log-magnitudes of the periodogram at the frequency corresponding to the maxima in the periodogram and the two neighboring points. Spectrogram plots were derived from the 90-d window moved in increments of 10 d across the mission (SI Appendix, SI Text). As is evident in the double-raster and spectral plots, all crewmembers except b had a predominant 24-h sleep–wake periodicity. Crewmember b had a sleep–wake period that varied between 24.72 and 25.06 h across the mission, increased with time in mission, and averaged 24.98 h for the entire mission. The smaller 24-h peak seen in the spectrogram of crewmember b was due to his daily attendance at breakfast between 08:00 and 10:00 each morning (SI Appendix, Table S3).