There are numerous stressors that are unique to the space flight environment that might account for sleep disturbances and prompt use of sleep-promoting medications. Noise, which can disrupt slow wave and REM sleep, both of which are critical to the restorative function of sleep, remains a major source of sleep disruption in modern space flight. Operational demands of space missions often result in the continuous noise levels in the range of 56-69 dBA and often higher within habitable areas of the International Space Station (ISS). Dosimetry measurements during ISS Expeditions 2-14 revealed noise levels during sleep episodes as high as 72 decibels. Researchers conducting laboratory-based studies have reported that noise levels similar to those experienced by astronauts during space missions could lead to sleep disruption and subsequently impaired cognitive functioning. Uncomfortable temperatures (too hot or too cold) contributed to disturbed sleep more than 20 percent of the time. Low levels of oxygen and high levels of carbon dioxide have been hypothesized to account for sleep disturbances during space flight. This study incorporated the existing extensive sleep database to search for correlations between environmental factors that NASA routinely records (i.e., hypoxia, noise, hypercapnia) and sleep.
The sleep-wake timing of 64 astronauts on 80 Space Shuttle missions, encompassing 26 Space Transportation System flights, and 21 astronauts on the ISS was objectively assessed, via wrist actigraphy and daily logs. Astronauts continuously wore an Actiwatch, a small, light-weight activity- and light-recording device throughout the space flight mission. Sleep duration was estimated in two-minute epochs from actigraphy data using Actiware Software. Astronauts also kept a daily sleep and medication log throughout their space flight mission. Subjective measures of sleep recorded in the diary included sleep quality and number of sleep disturbances.
The investigators have a database of over 4,000 sleep episodes, which provides a unique opportunity to objectively analyze other environmental factors that may influence sleep during space flight. The research team included NASA investigators and operational personnel who have collected time-stamped environmental data during space flight (e.g., noise levels, oxygen and carbon dioxide levels). Investigators statistically evaluated the association among objective and subjective measures of sleep quantity and quality with varying levels of these environmental factors.
None of the sleep variables showed any significant difference between the 14.7 psia and 10.2 psia conditions. There was a significant increase (22 minutes) in total sleep time (TST) when the mean partial pressure of carbon dioxide (ppCO2) was = 4 mmHg. When controlling for age, gender, and the reported use of sleep-promoting medications, one mmHg increase in the mean partial pressure of oxygen was associated with an increase of 58.8 minutes of TST, a 4.3% increase in sleep efficiency (SE), an increase 62.1 minutes in somatic symptom disorder (SSD), and a 15.3 unit increase in sleep quality (SQ). One mmHg increase in the mean ppCO2 was associated with a decrease of 8.8 minutes of TST and a decrease of 7.4 minutes in SSD. There were no personal acoustic dosimetry recordings at or above 80dBA during actigraphy-defined main sleep episodes; 94% of dosimetry recorded during main sleep episodes were less than 60dBA.