Sleep and Circadian Effects of Space
This paper describes how spaceflight impacts the quantity and quality of nightly sleep and how the misalignment of the sleep-wake schedule with the circadian timing system affects sleep and the use of fatigue countermeasures.
The duration, timing, and quality of sleep are critically important for physical health, mental health, performance, and safety (Committee on Sleep Medicine Research-Board on Health Sciences Policy 2006). During sleep, the brain’s neurons are maintained, repaired, and reorganized (Savage and West 2007), and toxic metabolic debris, generated and accumulated by neural activity during the waking day, is cleared (Nedergaard 2013; Xie et al. 2013). Insufficient sleep is associated with adverse metabolic and endocrine functioning (Spiegel et al. 1999), impaired immune response (Spiegel et al. 2002; Cohen et al. 2009), mood disturbances (Dinges et al. 1997), increased vulnerability to attentional failures (Dinges et al. 1997; Belenky et al. 2003; Van Dongen et al. 2003), obesity (Cappuccio et al. 2008), diabetes (Yaggi et al. 2006; Cappuccio et al. 2010; Holliday et al. 2013; Jackson et al. 2013), and cardiovascular disease (King et al. 2008; Cappuccio et al. 2010, 2011). Sleep also plays a crucial role in brain development, synaptic pruning, plasticity, rehearsal, memory consolidation, and learning (Roffwarg et al. 1966; Frank 2011; Bendor and Wilson 2012; Stickgold and Walker 2013; Tononi and Cirelli 2014; Vorster and Born 2015). A vast range of cognitive skills are impaired by sleep deficiency including vigilant attention, psychomotor speed, working memory, abstraction, and decisionmaking (Goel et al. 2013). For crewmembers who are required to maintain their physical and mental health and perform critical tasks flawlessly throughout increasing longer duration spaceflight missions, the ability to obtain adequate sleep is essential.
Early missions. The initial observation that humans could indeed sleep in space was made in 1961 on the Vostok-2 mission (Graeber 1987). Since that time, data have been sporadically collected on the sleep of crewmembers during spaceflight culminating in the largest objective study of sleep from 2002 to 2011. Graeber described the sleep of astronauts as often disturbed during early, relatively short flights of the American Mercury and Russian Vostok series. This was reportedly due to a combination of factors: the initial excitement of space flight, the noise disturbance, the effects of weightlessness, and the cramped quarters available for sleep (Graeber 1987).
Apollo. In later Apollo missions, poor sleep and subsequent crew fatigue were reported to have interfered with operational requirements (Berry 1969). According to a NASA technical report “The Apollo Medical Operations Project: Recommendations to Improve Crew Health and Performance for Future Exploration Missions and Lunar Surface Operations,” nearly all Apollo crews reported being tired on launch day and many reported sleep disruption through the missions. Some reported that their continuous sleep periods lasted no more than 3 h (Scheuring et al. 2007). According to one NASA report entitled, “Wide Awake on the Sea of Tranquility,” astronauts Neil Armstrong and Buzz Aldrin could not escape from light and noise in the tiny, cramped cabin of their spacecraft, and the spacesuit’s cooling system made it too cold for sleeping. Armstrong reportedly stayed awake all night, while Aldrin managed “a couple of hours of fitful drowsing” during their 21.6 h on the moon (NASA 2006). On one occasion, the command module pilot actually fell asleep while on duty, later resorting to amphetamine use to stay awake (Graeber 1987).
Skylab. Sleep was evaluated via electroencephalography (EEG), electrooculography (EOG), and head motion signals on crewmembers aboard the first two Skylab missions. In the 28 day mission, there was a decrease in total sleep time and an increase in stage 4 and REM sleep. In the 59 day mission, there was a small decrease in stage 4 and REM sleep, but no change in total sleep time. Crewmembers on both missions experienced sleep disruption in the final days of spaceflight with decreased REM latency postflight (Frost et al. 1975).
Russian Space Stations. During an extended duration Salyut mission, persistent insomnia led one cosmonaut to excessive hypnotic use, which was thought to have induced psychiatric symptoms. Cosmonauts on long-term missions appear to have been particularly vulnerable to the effects of fatigue, leading ground control to allow cosmonauts to sleep up to 12 h per day by the end of very long missions (Graeber 1987). Gundel and colleagues monitored sleep through diary and polysomnography for five nights during on a short stay on the MIR. Sleep duration inflight was reduced to 5.6 h, as compared to 6.4 during a terrestrial baseline measurement. Sleep architecture inflight was also changed such that REM and slow-wave sleep was redistributed, and REM latency was shortened (Gundel et al. 1993). The sleep of four MIR crewmembers was subsequently studied replicating the reduced inflight duration (6.1 h inflight vs. 6.4 h during baseline) and changes in sleep architecture (Gundel et al. 1997). Using the Nightcap sleep monitoring system (Ajilore et al. 1995), sleep recorded from 5 crewmembers for an average of 24 nights each during spaceflight revealed reductions in sleep duration, % REM, and sleep efficiency, as compared to preflight measures (Stickgold and Hobson 1999). One crewmember subjectively documented decreased sleep duration and increased sleep disturbance during a 5 month MIR mission (Monk et al. 2001).
Space Shuttle. The advent of the space shuttle led to improvement in sleeping conditions for crewmembers in space and light exposure protocols to preadapt them to their expected duty schedule in space in the week before launch. Nonetheless, a survey of 58 crewmembers from 9 shuttle missions revealed that most crewmembers suffered from sleep disruption during their missions and were only able to sleep an average of 6 h per day of flight as compared to 7.9 h each night on the ground (Santy et al. 1988). On STS-78, polysomnography was obtained from four crewmembers aboard a 17 day space shuttle mission (Monk et al. 1998). The results from both studies were consistent with prior reports showing reduced sleep duration and increased number of awakenings, even though the first and last nights of the mission (usually the most disturbed) were not recorded. Crewmembers reported losing an average of 2.2 h of sleep per night on the first and last days of flight (Santy et al. 1988). Four STS-89 crewmembers subjectively recorded sleep duration in log books revealing a 15% reduction in sleep duration as compared to terrestrial baseline data (Kelly et al. 2005).
In a later investigation spanning two space shuttle missions, Czeisler and colleagues evaluated polysomnographically recorded and subjectively reported sleep in four Neurolab crewmembers and one STS-95 crewmember. Compared to preflight, there was more wakefulness and less slow-wave sleep during the final third of sleep episodes during the spaceflight mission. Total sleep time during the spaceflight (6.5 h) was reduced compared to preflight (6.7 h) and postflight (6.8 h) measures. During spaceflight, actigraphy overestimated sleep by approximately one-half hour when compared to “gold standard” polysomnography. Additionally, crewmembers’ subjective assessment showed a greater number of awakenings and decreased sleep quality during space flight compared to preflight and postflight (Dijk et al. 2001).
To increase the scope of the investigation of sleep across multiple spaceflight missions, a large-scale actigraphic study of sleep was launched in 2001. Sixty-four crewmembers participated on 80 missions aboard 26 shuttle flights. The crewmembers wore an actigraph and completed daily diaries, resulting in 4173 nights of data collection, including 1063 nights during spaceflight. The duration of time astronauts attempted to sleep per night (time in bed assessed by diary) was significantly less, and it took them longer to fall asleep (sleep latency assessed by diary) on shuttle missions, during the 11 days before spaceflight and during the 2-week interval scheduled about 3 months before spaceflight than it was on their return to Earth. The mean duration of sleep during spaceflight was 6.0 h. Crewmembers slept on average 20 min longer each night during the 2-week interval scheduled about 3 months before spaceflight and 47 min longer each night in the first 7 days after landing, than they did during spaceflight. On about half of the nights aboard shuttle missions, crewmembers slept for fewer than 6 h, including nights before critical extravehicular activities. Subjective ratings of sleep quality and alertness were significantly higher in the first 7 days after landing than during the shuttle missions, in the 2-week interval scheduled about 3 months before spaceflight, and 11 days before spaceflight. Voids were the most commonly cited reason for sleep disturbance during all data collection intervals. Inflight, one in five sleep disruptions were attributed to noise, and one in four were attributed to unfavorable environmental temperature (e.g., too hot or too cold) (Barger et al. 2014). Of the 74 shuttle crewmembers who flew from 2005 to 2010 and participated in a retrospective survey and/or interview, one quarter reported not being able to fall asleep because of “thinking/active mind” and “anxiety about the mission,” or “upcoming tasks” was most often named as what the crewmembers were thinking about. A significant correlation was identified between scheduled workload and difficulty staying asleep. That is, the more demanding crewmembers perceived the schedule, the less likely they were to stay asleep during spaceflight (Whitmire et al. 2013).
International Space Station (ISS) . Sleep deficiency was evident in crewmembers aboard ISS missions nominally lasting 6 months. Twenty-one ISS crewmembers (n = 6 women) participated in the largest study of sleep on long-duration missions (Expedition 14  until Expedition 26 ), consisting of 4152 nights of actigraphy data collection, including 3248 nights during spaceflight. Astronauts slept significantly less during ISS missions (nightly average of 6.1 h) compared to terrestrial data collected approximately 3 months prior to flight (nightly average of 6.4 h) and the first week postflight (nightly average of 7.0 h). On 44% of nights aboard ISS missions, astronauts obtained less than 6 h of sleep. Subjectively, crewmembers reported, on average, sleeping 6.50 ± 0.73 each night inflight (Barger et al. 2014), overestimating the amount of sleep obtained by about 30 min per night. The subjective sleep duration data Barger and colleagues collected are consistent with subjective data collected from ISS crewmembers by Dinges and colleagues (2013). The objective data collected by Barger et al. reveal that such subjective sleep data likely overestimate the sleep actually obtained by crewmembers during spaceflight. That such sleep deficiency can be problematic was demonstrated by an analysis of ISS crewmembers’ journal entries. Sleep was among the top ten topic categories, and most of the entries were negative. Some examples of such entries include “Very tired. Woke up at 2 am and couldn’t get back to sleep. Finally fell asleep and overslept.”; “I fell asleep while typing.” and “I just need sleep” (Stuster 2010).
Sleep-promoting medications. With shifts in the sleep-wake times, busy schedules, noise, unfavorable temperatures, and needing to void as factors that disturb sleep in flight, it is not surprising that spaceflight crewmembers are often forced to resort to sleeping pill use in order to obtain sleep during their scheduled sleep episodes. In fact, sleeping-promoting medication has a long history of being used in the space program to address inflight sleep disturbances. Seconal, a short-acting barbiturate, was added to the inflight drug kit after the Apollo 7 crew reported sleep difficulties (Hawkins and Zieglschmid 1975). During nine early shuttle missions in 1985, 19% of the crewmembers on single shift missions and 50% of crewmembers on mission with dual shift operation reported sleeping pill usage during spaceflight (Santy et al. 1988). In a larger study of medication records from 79 shuttle missions, sleeping pills accounted for 45% of all medication use by astronauts in space. More doses of medication were taken for sleep than any other indication, with more than 500 unit doses administered (Putcha et al. 1999). The proportion of crewmembers using sleep-promoting drugs was even higher in the later years of the shuttle program and on ISS missions, even though all missions were single shift (i.e., all crewmembers had the same work and sleep schedule) and most ISS crewmembers had individual sleep stations. On shuttle missions from 2002 to 2011, 78% of shuttle crewmembers reported taking sleep-promoting drugs during spaceflight. The use of sleep drugs was reported on 52% of the inflight nights, with two doses of sleep drugs on 17% of nights on which such drugs were taken (Barger et al. 2014). A retrospective study corroborated these results with 71% of shuttle crewmembers indicated they used sleep medications to fall asleep during their spaceflight mission (Whitmire et al. 2013). Similarly, three-quarters of ISS crewmembers from 2006 to 2011 reported taking sleep-promoting drugs inflight on daily logs (Barger et al. 2014; Wotring 2015). An examination of medication use data from the Johnson Space Center Lifetime Surveillance of Astronaut Health was conducted for 24 crewmembers on ISS with an average mission duration of just over 5 months. Sleep-promoting medications were the most frequently used medication class during spaceflight; 71% of crewmembers reporting taking sleep-promoting medications (Wotring 2015). The reported incidence of sleep-promoting medication during spaceflight, 20 times greater than the proportion of Americans estimated to use hypnotic drugs at any time in a given year (Bertisch et al. 2014), was of little objective benefit. Although self-reported sleep latency and subjective sleep quality was slightly improved in crewmembers on nights when sleeping pills were taken, objective data of actigraphically recorded sleep aboard the shuttle show that, on average, sleep efficiency was increased by only 1.3%, without any significant improvement in sleep duration (Barger et al. 2014).
Contribution of circadian misalignment. Sleep deficiency during missions has been documented throughout spaceflight. Crewmembers attribute sleep disturbances to a number of environmental (e.g., noise, uncomfortable temperatures) and operational (e.g., scheduled workload) factors (Whitmire et al. 2013; Barger et al. 2014). Circadian misalignment is a physiological factor that contributes to sleep loss in shift workers and those who travel across time zones on Earth; crewmembers are at risk for circadian misalignment during spaceflight missions.
Light exposure is the primary synchronizer of human circadian rhythms to the 24 h day. Circadian misalignment occurs when the light exposure is improperly timed or not sufficiently intense. In low-Earth orbit, crewmembers are exposed to 90 min light-dark cycles, and the lighting on spacecraft is dim (Dijk et al. 2001). These factors and frequent operationally imposed shifts in the sleep-wake schedule can misalign the circadian system of crewmembers during spaceflight. When misaligned, crewmembers are attempting to sleep during the biological day when the body is programmed to be awake, and conversely, crewmembers need to be awake and perform their jobs when the body is programmed to be asleep.
On an 8 day MIR mission, the circadian phase of the core body temperature rhythm in one astronaut delayed 2–3 h with no change in amplitude (Gundel et al. 1993). Subsequently, three of four crewmembers exhibited 2 h phase delays and reduced amplitude in the core body temperature rhythm during a MIR mission, as compared to baseline (Gundel et al. 1997). A case study of a crewmember’s oral temperature rhythm during a 100 day MIR mission showed proper circadian alignment during the first two-thirds of the mission with apparent drift and reduced amplitude in the final third (Monk et al. 2001).
Due to orbital mechanics, crewmembers’ sleep episodes were often progressively scheduled to an earlier hour during shuttle missions, thereby enabling 4–7 h phase advance shifts either gradually, 20–40 min earlier per day, or in a series of 2 h phase jumps. This type of schedule is one of the most effective laboratory means of inducing sleep-onset insomnia (Fookson et al. 1984; Strogatz et al. 1987). Despite the circadian challenge of a 25 min per day advance shift throughout a shuttle mission, the circadian rhythms of four crewmembers were very similar in phase and amplitude during spaceflight and during terrestrial measurements (Monk et al. 1998). In another study of five crewmembers aboard two later shuttle flights, circadian rhythms of cortisol did not shift in conjunction with the scheduled 20 min daily advance in sleep timing, resulting in circadian misalignment. Using actigraphy, photometry, and log data that were collected from 21 crewmembers over 3248 days of long-duration spaceflight on the ISS, estimates of circadian phase were determined via a mathematical model. The crewmembers’ estimated endogenous circadian temperature minimum occurred outside sleep episodes and was thus deemed misaligned, on 19% of sleep episodes during spaceflight, and the circadian misalignment was significantly associated with 1 h loss of total sleep time, and a higher prevalence of use of sleep-promoting medications (Flynn-Evans et al. 2016).
Outlook for sleep on future exploration mission. On future exploration missions, crewmembers maintaining circadian alignment and obtaining adequate sleep will become even more important as the day length on other planets varies (Barger et al. 2012), and the duration of the spaceflight mission increases (Basner et al. 2013).
In order to maintain the health, performance, and safety of crewmembers, it is essential that they obtain sleep of sufficient duration and quality (Committee on Sleep Medicine Research- Board on Health Sciences Policy 2006). The high prevalence of sleep-promoting medication used during spaceflight, notwithstanding chronic sleep deficiency, suggests research on environmental aspects of the space environment, such as microgravity itself, should continue in order to determine what contributes to the sleep disturbances long associated with spaceflight. The development of other effective countermeasures to promote sleep and proper circadian alignment during spaceflight is crucial. These countermeasures might include scheduling modifications, strategically timed exposure to specific wavelengths and intensities of light and behavioral strategies to ensure adequate sleep.
- Barger L, Sullivan JP, Vincent AS, Fiedler ER, McKenna LM, Flynn-Evans EE, Gilliland K, Sipes WE, Smith PH, Brainard GC, Lockley SW (2012) Learning to live on a Mars day: fatigue countermeasures during the Phoenix Mars Lander mission. Sleep 35(10):1423–1435Google Scholar
- Basner M, Dinges DF, Mollicone D, Ecker A, Jones CW, Hyder EC, Di Antonio A, Savelev I, Kan K, Goel N, Morukov BV, Sutton JP (2013) Mars 520-d mission simulation reveals protracted crew hypokinesis and alterations of sleep duration and timing. Proc Natl Acad Sci U S A 110(7):2635–2640CrossRefGoogle Scholar
- Berry CA (1969) Preliminary clinical report of the medical aspects of Apollos VII and VIII. Aerosp Med 40:245–254Google Scholar
- Committee on Sleep Medicine Research- Board on Health Sciences Policy (2006) Sleep disorders and sleep deprivation: an unmet public health problem. Institute of Medicine of the National Academies; The National Academies Press, Washington, DCGoogle Scholar
- Dijk DJ, Neri DF et al (2001) Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights. Am J Phys Regul Integr Comp Phys 281:R1647–R1664Google Scholar
- Dinges DF, Pack F et al (1997) Cumulative sleepiness, mood disturbance, and psychomotor vigilance performance decrements during a week of sleep restricted to 4–5 hours per night. Sleep 20(4):267–277Google Scholar
- Dinges D, Basner MDJM, Goel N, Braun M (2013) ISS missions: elevated workload and reduced sleep duration. NASA Human Research Program Investigators’ Working Group, Galveston. [Abstract]Google Scholar
- Fookson JE, Kronauer RE et al (1984) Induction of insomnia on a non-24 hour sleep–wake schedule. Sleep Res 13:220. [Abstract]Google Scholar
- Graeber RC (1987) Sleep in space. In: Roussel B, Jouvet M (eds) Proceedings of the XXVII NATO DRG seminar sleep and its applications for the military. A.C.E.M.L., Lyon, pp 59–69Google Scholar
- Hawkins W, Zieglschmid J (1975) Clinical aspects of crew health. In: Dietlein LF, Johnston RS, Berry CA (eds) Biomedical results of Apollo. Johnson Space Center, Washington, DC, pp 43–82Google Scholar
- National Aeronautics and Space Administration (2006) Wide awake on the sea of tranquility, July 20, 2006. http://www.nasa.gov/exploration/home/19jul_seaoftranquillity.html. Accessed 12 Apr 2013
- Putcha L, Berens KL et al (1999) Pharmaceutical use by U.S. astronauts on space shuttle missions. Aviat Space Environ Med 70(7):705–708Google Scholar
- Santy PA, Kapanka H et al (1988) Analysis of sleep on shuttle missions. Aviat Space Environ Med 59:1094–1097Google Scholar
- Scheuring RA, Jones JA et al (2007) The Apollo medical operations project: recommendations to improve crew health and performance for future exploration missions and lunar surface operations. National Aeronautics and Space Administration. NASA/TM-2007-214755, Houston, pp 1–452Google Scholar
- Stickgold R, Hobson JA (1999) REM sleep and sleep efficiency are reduced during space flight. Sleep 22(Suppl):S82Google Scholar
- Strogatz SH, Kronauer RE, Czeisler CA (1987) Circadian pacemaker interferes with sleep onset at specific times each day: role in insomnia. Am J Phys 253:R172–R178Google Scholar
- Stuster J (2010) Behavioral issues associated with long-duration expeditions: review and analysis of astronaut journals. Johnson Space Center, HoustonGoogle Scholar
- Whitmire A, Slack K, Locke J, Keeton K, Patterson H, Faulk J, Leveton L (2013) Sleep quality questionnaire short-duration flyers. National Aeronautics and Space Administration, Johnson Space Center, HoustonGoogle Scholar