Sleep in space as a new medical frontier: the challenge of preserving normal sleep in the abnormal environment of space missions

Space agencies such as the National Aeronautics and Space Administration of the United States, the Russian Federal Space Agency, the European Space Agency, the China National Space Administration, the Japan Aerospace Exploration Agency, and Indian Space Research Organization, although differing in their local political agendas, have a common interest in promoting all applied sciences that may facilitate man’s adaptation to life beyond the earth. One of man’s most important adaptations has been the evolutionary development of sleep cycles in response to the 24 hour rotation of the earth. Less well understood has been man’s biological response to gravity. Before humans ventured into space, many questioned whether sleep was possible at all in microgravity environments. It is now known that, in fact, space travelers can sleep once they leave the pull of the earth’s gravity, but that the sleep they do get is not completely refreshing and that the associated sleep disturbances can be elaborate and variable. According to astronauts’ subjective reports, the duration of sleep is shorter than that on earth and there is an increased incidence of disturbed sleep. Objective sleep recordings carried out during various missions including the Skylab missions, space shuttle missions, and Mir missions all support the conclusion that, compared to sleep on earth, the duration in human sleep in space is shorter, averaging about six hours. In the new frontier of space exploration, one of the great practical problems to be solved relates to how man can preserve “normal” sleep in a very abnormal environment. The challenge of managing fatigue and sleep loss during space mission has critical importance for the mental efficiency and safety of the crew and ultimately for the success of the mission itself. Numerous "earthly" examples now show that crew fatigue on ships, trucks, and long-haul jetliners can lead to inadequate performance and sometimes fatal consequences, a reality which has caused many space agencies to take the issue of sleep seriously.