Circadian challenge of astronauts\u2019 unconscious mind adapting to microgravity in space, estimated by heart rate variability

Kuniaki Otsuka,Satoshi Furukawa,Hiroshi Ohshima,Mitsutoshi Hayashi,Chiaki Mukai,Germaine Cornelissen,Yutaka Kubo,Koichi Shibata,Koh Mizuno

doi:10.1038/s41598-018-28740-z

Abstract

It is critical that the regulatory system functions well in space’s microgravity. However, the “intrinsic” cardiovascular regulatory system (β), estimated by the fractal scaling of heart rate variability (HRV) (0.0001–0.01 Hz), does not adapt to the space environment during long-duration (6-month) space flights. Neuroimaging studies suggest that the default mode network (DMN) serves a broad adaptive purpose, its topology changing over time in association with different brain states of adaptive behavior. Hypothesizing that HRV varies in concert with changes in brain’s functional connectivity, we analyzed 24-hour HRV records from 8 healthy astronauts (51.8 ± 3.7 years; 6 men) on long (174.5 ± 13.8 days) space missions, obtained before launch, after about 21 (ISS01), 73 (ISS02), and 156 (ISS03) days in space, and after return to Earth. Spectral power in 8 frequency regions reflecting activity in different brain regions was computed by maximal entropy. Improved β (p < 0.05) found in 4 astronauts with a positive activation in the “HRV slow-frequency oscillation” (0.10–0.20 Hz) occurred even in the absence of consciousness. The adaptive response was stronger in the evening and early sleep compared to morning (p = 0.039). Brain functional networks, the DMN in particular, can help adapt to microgravity in space with help from the circadian clock.

Highlights

As humans venture into space, it is critical that the regulatory system should remain fully functional
In 2 of them, heart rate variability (HRV)-VSFO activation occurred during ISS03, in another it occurred during both ISS02 and ISS03, and in another one it occurred only during ISS01
Neuroimaging studies suggest that HRV varies in concert with changes in brain functional connectivity[34,68,69,70,71,72,73,74]

Summary

Introduction

As humans venture into space, it is critical that the regulatory system should remain fully functional. Humans, including their cardiovascular system must first acclimate to a new environment in order to survive. Baroreflex sensitivity fluctuates and blood volume distribution is altered, affecting neural mechanisms involved in dynamic cardiovascular coordination. The “intrinsic” cardiovascular regulatory system, estimated by the fractal scaling of heart rate variability (HRV) (slope “β”), did not adapt to microgravity after 6 months in space[10,11,12], whereas the circadian rhythm of heart rate (HR) did improve[10], an important feature since circadian disruption adversely affects health[13,14,15,16,17]. The ECN not rarely induces a shift of DMN's signal from may involve balancing internally-oriented DMN activity

Methods

Results

Conclusion