Astronauts suffer considerable sleep deficiency in the weeks leading up to and during spaceflight, according to the most extensive study of sleep during spaceflight ever conducted, published in The Lancet Neurology journal.
Fatigue and sleep deficiency are common complaints among astronauts, but this is the most comprehensive study to include both objective evaluation of sleep (via an actigraph, a device worn on the wrist which records sleep and wake cycles) and subjective evaluations (via a daily diary recording alertness and sleep quality).
Researchers from Brigham and Women's Hospital, Harvard Medical School, Boston, USA, and the University of Colorado, Boulder, USA, studied the sleep patterns of 64 astronauts on 80 Shuttle missions and 21 astronauts aboard International Space Station (ISS) missions before, during, and after spaceflight. In total, they recorded more than 4000 nights of sleep on Earth and more than 4200 in space.
Despite NASA scheduling 8•5 hours of sleep per night for crew members in spaceflight, the average (mean) duration of sleep during spaceflight was just under six (5•96) hours on shuttle missions and just over six hours (6•09) on ISS missions. 12% of sleep episodes on shuttle missions and 24% on ISS missions lasted 7 hours or more, as compared with 42% and 50%, respectively, in a post-flight data collection interval when most astronauts slept at home.
Moreover, the results suggest that astronauts' build-up of sleep deficiency began long before launch, as they averaged less than 6•5 hours sleep per night during training (recorded approximately 3 months prior to spaceflight.) This is about half an hour less per night than the average American adult.
According to Dr Laura K. Barger from Brigham and Women's Hospital, Harvard Medical School, Boston, USA, "Sleep deficiency is pervasive among crewmembers. It's clear that more effective measures are needed to promote adequate sleep in crew members, both during training and spaceflight, as sleep deficiency has been associated with performance decrements in numerous laboratory and field-based studies."*
The research also highlights widespread use of sleeping medication such as zolpidem and zaleplon during spaceflight. Sleep medication use was reported by three quarters of ISS crewmembers at some point during their time on the space station, and by more than three quarters (78%) of shuttle-mission crew members. Sleep medication was used on more than half (52%) of nights in shuttle missions.
"The ability for a crewmember to optimally perform if awakened from sleep by an emergency alarm may be jeopardized by the use of sleep-promoting pharmaceuticals," says Dr Barger. "Routine use of such medications by crewmembers operating spacecraft are of particular concern, given the FDA warning that patients using sleeping pills should be cautioned against engaging in hazardous occupations requiring complete mental alertness or motor coordination, including potential impairment of the performance of such activities that may occur the day following ingestion of sedative/hypnotics. This consideration is especially important because all crewmembers on a given mission may be under the influence of a sleep-promoting medication at the same time. In fact, on the four shuttle missions on which all crewmembers participated, all crewmembers reported taking sleep-medications on the same night 6% of the time."
The spaceflight environment, where the sun rises and sets every 90 minutes, is particularly harsh for sleep. Apollo astronauts cited light, noise, and the cooling systems in spacesuits as contributing factors to their poor sleep. However, sleep disturbances continue to occur in modern spaceflight despite quiet and dark 'sleep stations' installed on the ISS, leading some scientists to speculate that microgravity itself may be to blame.
Co-author Dr Charles Czeisler of Brigham and Women's Hospital added, "Future exploration spaceflight missions to the moon, Mars, or beyond will require more effective countermeasures to optimise human performance by promoting sleep during spaceflight. These may include modifications to schedules, strategically timed exposure to specific wavelengths of light, and behavioural strategies to ensure adequate sleep, which is essential for maintaining health, performance and safety."*
Writing in a linked comment, Dr Mathias Basner from the University of Pennsylvania, USA, points out the need for further research on the effects of space on sleep. "Only four people have consecutively lived and worked for more than 1 year in space; as such, how sleep and behavioural health will be affected during space exploration is poorly understood. Studies of the physiology of sleep stages and the intensity of sleep...in space are necessary to answer the important question of whether spaceflight reduces the need for sleep and therefore the ability to sleep, or whether it reduces the ability to sleep but not the need for sleep."
This study was funded by NASA and the National Space Biomedical Research Institute (NSBRI).