Ever since the Apollo 11 headed to the Moon in late 1969, astronauts have complained of sleep deprivation. Now a study has shed light on the extent of the sleep deprivation and fatigue suffered by those who travel into space.

In accordance with official NASA flight schedules, astronauts are ordered to get 8.5 hours of sleep every night. But after tracking the sleep patterns of 85 crew members aboard the International Space Station (ISS) and space shuttles, researchers have discovered that astronauts on shuttle missions sleep for under six hours per night on average and just over six on ISS missions.

Crew members on modern space missions sleep in quiet, darkened chambers, and three quarters of astronauts take sedatives—yet the problem still prevails. It was not only that astronauts failed to get the required amount of sleep, however. In the three-month pre-flight training period, sleep was also found to be significantly disturbed. During this time crew began to build up a long-term sleep deficiency, averaging less than 6.5 hours while in training.

The number of astronauts taking sleep medication has also been highlighted as a particular concern, after the study discovered that three-quarters of ISS crew take sleep medication at least once during their stay and that 78 percent of shuttle crew members used medication on more than half of their nights in space.

"The ability for a crew member to optimally perform if awakened from sleep by an emergency alarm may be jeopardized by the use of sleep-promoting pharmaceuticals," warned Laura K Barger from Brigham and Women's Hospital, who is lead study author. Taking sedatives and hypnotics has also widely been found to impair performance and physical activities, which is why many such medications carry warnings against operating heavy machinery or vehicles after ingestion.

The ten-year study has seen more than 4,000 nights of astronauts' sleep on Earth and over 4,200 nights of sleep in space be recorded and has concluded that more action needs to be taken to help optimize performance of astronauts.

"Because chronic sleep loss leads to performance decrements, our findings emphasize the need for development of effective countermeasures to promote sleep," write the authors in the study's abstract.

Sleep deprivation has been found to be highly detrimental to an astronaut's performance, both during training and space flight. Closer to Earth, around 15 to 20 percent of fatal airline crashes caused by human error are thought to be a direct result of pilot fatigue. There is no consensus yet on what countermeasures should be implemented, but they could include scheduling modifications and the teaching of behavioral strategies.