How does microgravity affect an Astronauts sleep-wake cycle on the space station as compared to sleeping on Earth Including how fluid shifts can lead to sleep disturbances.
By David Barckhoff University of Pittsburgh
Microgravity significantly alters and impacts an astronaut’s sleep-wake cycle on the International Space Station (ISS) compared to Earth. This is primarily due to the absence of a gravitational pull that influences shorter sleep durations, fluid distribution, circadian rhythms, and altered sleep architecture. On Earth, gravity plays a crucial role in maintaining hydrostatic pressure, which affects blood flow and fluid shifts within the body. In microgravity, this fundamental force is absent, leading to a cephalic fluid shift where fluids migrate from the lower extremities towards the head, neck, and chest. These effects are influenced by a combination of environmental, physiological, and operational factors unique to the space environment [1] [2] [3] [4].
Sleep Duration and Quality
Astronauts consistently experience reduced sleep duration in space. Studies have shown that astronauts on the International Space Station (ISS) and during Space Shuttle missions average around 6 hours of sleep per day, which is significantly less than their pre-flight sleep on Earth (typically 7-8 hours) [7] [11] [12]. For instance, a study of 64 Space Shuttle astronauts and 21 ISS astronauts found average daily sleep times of 5.96 hours and 6.09 hours, respectively, compared to longer durations post-mission [7]. Some astronauts even reported sleeping less than 5 hours on certain nights [11]. This chronic partial sleep loss can lead to cognitive impairment similar to that experienced from total sleep loss [1].
Beyond just duration, the quality and architecture of sleep are also negatively affected. Research on Mir space station cosmonauts revealed shorter sleep durations, increased wakefulness, and a 17.7% reduction in sleep efficiency [10]. The amount of non-REM and REM sleep decreased by 14.1% and 25.8% respectively, and it took nearly 1.5 times longer to reach the first REM sleep episode in space compared to Earth [10]. Another study on Mir astronauts found REM sleep time reduced by 50% and overall sleep time reduced by 27% compared to pre-flight [11].