Warning that insomnia could be a major problem in space, a new study said that the widespread use of sleeping pills by astronauts can make them less vigilant in the high risk environment of space.
NASA-funded researchers collected data from 64 astronauts involved in 26 flights on the US space shuttle, and 21 others from 13 missions to the International Space Station (ISS).
A wrist-worn device called an actigraph monitored sleep and wake cycles, and the astronauts kept a daily log of their alertness and sleep quality.
In all, more than 4,000 nights of sleep on Earth and 4,200 in space were recorded, making this the most extensive investigation yet of shut-eye in orbit.
The data pointed to a chronic lack of sleep among space travellers which begins during pre-flight training, about three months prior to launch.
On average, astronauts slept just under six hours a night on both shuttle flights and ISS missions -- far short of NASA's 8.5-hour guideline.
Just 12 percent of "sleep episodes" on shuttle missions and 24 percent on ISS missions lasted seven hours or more, the research found.
When the astronauts returned home, their seven-hour-plus sleeps rose to 42 and 50 percent respectively.
"Sleep deficiency is pervasive among crewmembers," said Laura Barger from Brigham and Women?s Hospital at the Harvard Medical School in Massachusetts.
"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."
- Crew performance 'jeopardised' -
The paper, published in The Lancet Neurology, found that three-quarters of astronauts used standard sleeping aids like zolpidem (whose brand names include Stilnox and Ambien) and zaleplon (marketed under names like Sonata and Andante) while in space.
On one night in six of using sleep drugs, astronauts took a double dose to try to get some shut-eye.
On shuttle missions, medication was used on more than half the nights. On four of the 13 shuttle missions, all crew members took sleeping drugs on the same night six percent of the time.
Routine use of these drugs is "of particular concern," given their potential side-effects on mental alertness and motor coordination, said Barger.
"The ability for a crew member to optimally perform if awakened from sleep by an emergency alarm may be jeopardised by the use of sleep-promoting pharmaceuticals,? she said.
Orbital spaceflight -- where the Sun rises and sets every 90 minutes -- is a tough environment for sleep.
Astronauts, who used sleeping bags on the shuttle and individual quarters on the ISS, cited light and noise among the sleep hindrances.
But sleep disturbance continued after the ISS was fitted with quiet, dark "sleep stations", suggesting the problem may lie with the gravity-free conditions of space, the researchers said.
Pointing to plans for long-duration flights to Mars, the study said it was imperative to find ways to boost sleep in space.
Changes to behavioural routines and work schedules, and "strategically-timed" exposure to specific wavelengths of light, may encourage the right kind of drowsiness, it said.
The probe did not include Russian cosmonauts due to differences in US and Russian policies for participation in research, the paper said.