Space Flight Dramatically Reduces Sleep-Related Disturbances and Snoring

San Diego, CA - August 20, 2001

Just as the shuttle Discovery returns this week from a mission to the International Space Station bringing back three astronauts and two tons of trash, a study appears that discusses the impact of space flight on sleep-disordered breathing (SDB), a study done with five astronauts.

Sleep Quality in Space

Both the American and the Russian space programs have documented sleep disruption and reduced amounts of sleep during space flights, but the cause of sleep disruptions is still not known. Since the astronauts and cosmonauts need to perform at high levels during the flights, sleep hypnotics are the most commonly used medications during space flight, with as many as half of the astronauts using hypnotics on any given mission. One study indicated that 45% of all medications used by 219 astronauts through 79 shuttle missions were hypnotics taken for sleep disturbance. While NASA tries to allow eight hours of sleep per day for astronauts, fifteen years of reports indicate that sleep duration of about six hours is the norm in space.

G. Kim Prisk, Ph.D., of the Department of Medicine, University of California, San Diego, with 8 associates, studied the effect of space flight on sleep-disordered breathing before, during, and after 2 National Aeronautic and Space Administration (NASA) space shuttle missions in 1998. Their results were recently published in the first issue for August of the American Thoracic Society's peer-reviewed American Journal of Respiratory and Critical Care Medicine, Vol. 164, pp. 478-485.

Their study was undertaken to determine if sleep-disordered breathing was the cause of the sleep disruptions. The initial premise was that the lack of gravity would tend to lessen the number of SDB events, as the action of gravity on the upper airway structures is thought to have a major impact on SDB. On the other hand, the lack of gravity would also increase the amount of blood and body fluids in the head and upper body, possibly effecting respiratory mechanics, since increased fluid volume in the head and neck could reduce the size of the upper airway. There had been no significant study of sleep during space flight before this experiment.

Gathering the Data

The investigators studied 77 8-hour, full polysomnographic recordings from the 5 healthy subjects, including 9 studies before the space flight, on 4 occasions per subject during either a 16- or 9-day shuttle mission, and on the second, fourth and fifth sleep periods after their return to earth. One astronaut received only six pre-flight studies.

Since there was no way to put a full sleep center into the shuttle, all sleep studies were done with specially designed equipment. This included a portable digital sleep recorder, a custom fitted sleep cap (with electrodes integrated into the cap to insure consistent placement), and a special body suit (vest and shorts) that contained the chest and abdominal bands and other sensors. Even the sleep studies done pre-flight and post-flight were done utilizing the same equipment to insure consistency in results. During space flight, astronauts were partnered to insure that the equipment checked out before going to sleep.

Study Results

In the microgravity of space flight, these five astronauts showed dramatic reductions in the number of sleep-related disturbances, time spent snoring, and the number of arousals associated with respiratory-related events. The results show that sleep-disordered breathing does not cause any sleep disruption in space.

The investigators believe their study is the first direct demonstration that gravity plays a dominant role in the generation of apnea, hypopneas, and snoring in healthy persons.

Microgravity in space was associated with a 55 percent reduction in the apnea-hypopnea index (AHI) which decreased from an average preflight value of 8.3 events per hour for the 5 astronauts to 3.4 events per hour during sleep in space. This reduction in AHI was accompanied by a virtual elimination of snoring which fell from 16.5 percent of total sleep time preflight to 0.7 percent in-flight. A decrease in respiratory arousals in space amounted to a reduction of 19 percent from preflight levels.

Three of the 4 men and 1 woman (average age 41) who participated had preflight AHI indices that were below 5 events per hour, classifying them as not having any sleep-related breathing disturbances. Hypopneas were the primary classification for their respiratory disturbances. A fourth astronaut had 6.1 events per hour of sleep preflight and the fifth 22.7 per hour.

According to Dr. Prisk, the respiratory system is greatly influenced by the force of gravity. Significant changes take place when an individual goes from standing upright to lying on their back (supine). In the supine position, functional residual capacity, expiratory reserve, and tidal volume are all reduced, affecting lung volume. This, in turn, reduces upper airway diameter. In the supine position, gravity works to reduce upper airway size and to increase upper airway resistance by causing the tongue, soft palate, uvula, and epiglottis to move backward toward the posterior pharyngeal wall.

The investigators believe the supine position alone may be one of the primary causes of the mildest form of upper airway resistance syndrome.

The authors note their results during sleep in space are probably due to the passive elimination of gravitationally induced changes in the upper airway anatomical structures.

In an accompanying editorial, Dr. David Dinges of the University of Pennsylvania suggests that perhaps the improvement in sleep-disordered breathing and the reduction in brain arousals in these essentially normal individuals may help to consolidate sleep and improve sleep efficiency in these individuals, leading to a reduced need for sleep quantity. This is a phenomenon also reported anecdotally by many sleep apnea patients on Earth, who report better quality sleep, but less of it, as they settle into use of CPAP to treat their sleep apnea.