當太空人睡於外太空之太空梭內時,其心跳較地面時低(地心引力減少,生理負擔較小之故),HRV研究發現係副交感增強之故
Heart period and heart period variability during sleep on the MIR space station.
J Sleep Res. 1999; 8(1):37-43 (ISSN: 0962-1105)
Gundel A; Drescher J; Spatenko YA; Polyakov VV
DLR Institute of Aerospace Medicine, Cologne, Germany.

The long-term acclimation of cardiac rhythms to microgravity was studied in four astronauts aboard the Russian space station MIR during wakefulness and sleep. Sleep polygraphies were obtained between the third and the 30th day in space and, in addition, prior to mission on the ground. From each of the sleep polygraphies, beat-to-beat intervals of cardiac rhythms were determined. The response of heart period and heart period variability to the stimulus microgravity was tested during sleep across sleep stages and during waking. A lengthening of heart period by about 100 ms was found in space compared to measurements on the ground. The slowing of heart rate was more pronounced for non-REM sleep than for REM sleep. A systematic change in heart period in relation to the duration of the stay in space could not be detected. An analysis of heart period variability in the high frequency (respiratory sinus arrhythmia) band supports the hypothesis that the decrease of heart rate under microgravity is produced by an increase in parasympathetic activity. Testing the response of cardiac rhythms to microgravity across distinct behavioural states seems to be a powerful tool to investigate the cardiovascular system.