Content uploaded by Zachary A Caddick
Author content
All content in this area was uploaded by Zachary A Caddick on Feb 25, 2019
Content may be subject to copyright.
Sleep Environment Recommendations for Future
Spaceflight Vehicles
Zachary A. Caddick1, Kevin Gregory1, and Erin E. Flynn-Evans2
1San Jose State University Research Foundation, San Jose CA, USA and
2NASA Ames Research Center, Moffett Field CA, USA {erin.e.flynn-evans@nasa.gov}
Abstract. Evidence from spaceflight and ground-based missions demonstrate that
sleep loss and circadian desynchronization occur among astronauts, leading to
reduced performance and, increased risk of injuries and accidents. We conducted a
comprehensive literature review to determine the optimal sleep environment for
lighting, temperature, airflow, humidity, comfort, intermittent and erratic sounds,
privacy and security in the sleep environment. We reviewed the design and use of
sleep environments in a wide range of cohorts including among aquanauts,
expeditioners, pilots, military personnel, and ship operators. We also reviewed the
specifications and sleep quality data arising from every NASA spaceflight mission,
beginning with Gemini. We found that the optimal sleep environment is cool, dark,
quiet, and is perceived as safe and private. There are wide individual differences in
the preferred sleep environment; therefore modifiable sleeping compartments are
necessary to ensure all crewmembers are able to select personalized configurations
for optimal sleep.
Keywords: Extreme Environments · Habitability · Human Factors · Sleep
1 Introduction
Sleep quality -- including the ability to fall asleep and remain asleep -- and sleep duration
are dependent upon circadian phase, length of prior wake duration, and time within the
sleep episode [1-3]. Proper alignment of scheduled sleep episodes to the circadian
pacemaker is important for sleep consolidation and sleep structure [4-5]. High sleep
efficiency is best maintained for eight hours when sleep is initiated approximately six
hours before the endogenous circadian minimum of core body temperature [4-5]. This
phase relationship between the rest-activity cycle and the endogenous circadian timing
system implies that even small circadian phase delays of the sleep propensity rhythm with
respect to the rest-activity schedule can result in sleep onset insomnia or substantial wake
after sleep onset.
In order to quantify the impact of a sub-optimal sleep environment on sleep quality
and duration, it is important to measure sleep outcomes when sleep is appropriately timed
relative to the circadian and homeostatic drives for sleep. It is possible for an individual to
experience sleep disruption in an optimal sleep environment due to the imposed sleep
schedule. Similarly, it is possible for an individual to experience high sleep efficiency in a
sub-optimal sleep environment when accumulated sleep debt is present, which dampens
the arousal threshold. Our aim was to compile the evidence associated with sleep
disruption due to controllable, environmental stimuli in order to aid NASA engineers and
operational personnel in the optimal design of crew sleep accommodations for deep
spaceflight.
2 Methods
We conducted a comprehensive literature review summarizing optimal sleep hygiene
parameters for lighting, temperature, airflow, humidity, comfort, intermittent and erratic
sounds, privacy and security in the sleep environment. We reviewed the design and use of
sleep environments in a wide range of cohorts including among aquanauts, expeditioners,
pilots, military personnel and ship operators. We also reviewed the specifications and
sleep quality data arising from every NASA spaceflight mission, beginning with Gemini.
3 Recommendations
The sleep environment required for long duration missions will differ from the sleep
accommodations that NASA has developed in the past. Our review revealed several
modifications that will be important to make in order to ensure that deep space crews have
sleep environments that will provide them with quality sleep.
3.1 Sleep Chamber Location
The location of the sleep station within the vehicle is key to reducing noise and light
pollution. Noise emanating from common areas has been shown to be disruptive to sleep
[6-7]. Given that there are individual differences in sleep timing preference, it is likely
that some crew will chose to be awake, while others are asleep [8-9]. In order to ensure
that morning-types and evening-types are both afforded adequate rest, it is desirable to
position crew quarters away from the galley area and exercise machinery. We also found
that individuals living in a variety isolated and confined environments reported
experiencing sleep disruption due to other crewmembers using the waste management
system during sleep episodes [9-11]. Therefore, the waste management system should be
located far enough away from sleeping quarters that noise is buffered, but close enough
that crewmembers are able to quickly access the facility and return to sleep without
having to travel too far. It may be appropriate to locate waste management facilities in a
module adjacent to the sleep stations.
It is likely that watch schedules will be necessary during deep space missions. We
found that in the early history of human spaceflight, watch schedules were very disruptive
to sleeping crewmembers due to the close proximity of the sleeping crewmember to the
“on watch” crewmember [12]. According to studies of military personnel and pilots,
locating the sleep chambers for off-duty crewmembers away from the command and
communication area is desirable [11, 13-15]. However, the sleep chambers should be
positioned near enough to the vehicle command center that crewmembers may quickly
respond in an emergency situation [11].
3.2 Privacy
It is imperative that each crewmember is provided with a private sleep chamber for the
duration of the mission. We found that shared sleep spaces and common bunkrooms are
associated with frequent sleep disruption due to other crewmembers [13]. The practice of
“hot bunking” has been virtually eliminated from all occupations that we evaluated due to
hygiene concerns and the impact that hot bunking has on psychological mood and health
[13, 16]. We found that individuals view their sleep location not just as a place for sleep,
but also as a space for privacy [7, 14, 16-25]. Access to a private space is viewed as
critical to the psychological well-being of individuals living in isolated and confined
environments [26]. Similarly, provision for storage of personal items within the sleep
chamber was viewed as highly desirable [9, 27]. The sleep chambers for deep space
vehicles should also allow crewmembers to customize the space with personal items and
reconfiguration of stowage compartments [9].
There have been situations where crewmembers have been displaced from private
quarters during spaceflight missions [28]. In these situations it is very difficult for the
displaced individuals to obtain adequate sleep [29-31]. Given that the loss of a sleep
chamber would likely also be associated with a breach of the spaceflight vehicle, the
resulting anxiety may further reduce crewmember sleep quality and quantity. As a result,
it is possible that the loss of a sleep chamber could greatly impact the physical and
psychological health of crewmembers at a time when successful performance of duties is
essential. Given the importance of sleep in conferring fitness for duty, future crew
vehicles should include back up, deployable sleep chambers in order to ensure that
individuals have access to a private sleep environment throughout the mission.
3.3 Habitable Volume
The crew quarters that are presently on ISS appear to provide enough habitable volume
for crewmembers to move as desired during sleep [32-33]. We found one case where a
crewmember was too large to fit in the assigned sleep chamber during spaceflight [34].
Although it may be necessary to design all sleep chambers and sleeping bags to the same
standard, it is important to consider that larger crewmembers will have less habitable
volume relative to smaller crewmembers. As such, it is important to ensure that the
crewmembers selected for a deep space mission are able to evaluate the size of the sleep
stations in advance of the mission. It may also be desirable to design two sizes for the
sleep stations to accommodate larger and smaller crewmembers.
The optimal sleep environment for a planetary excursion will be necessarily different
from the optimal sleep environment for spaceflight. During a long duration planetary
excursion, larger crew quarters are necessary due to the comparatively reduced habitable
space available in a partial gravity environment. We found that individuals living in
isolated and confined environments on Earth use their sleep rooms as a place for privacy
and to work in addition to sleep [7, 27, 35]. As a result, the crew rooms on a planetary
excursion should include space for a bed (placed horizontally on the floor), a desk and
storage of personal belongings. The use of bunkrooms or shared sleep spaces is only
appropriate for a short-duration planetary excursion. In these cases, bunks or cots may be
used to accommodate crewmembers [7]; however, even during such short excursions
private crew quarters would be preferable [27].
3.4 Light
Sleep chambers in spaceflight and on the ground must include features that protect
individuals from being awoken by external forces such as light, noise, inadequate
temperature and poor air quality. Light is the primary resetting cue for the human
circadian pacemaker [36]. Exposure to light at inappropriate times leads to circadian
misalignment, which causes sleep disruption [37]. Similarly, exposure to light is alerting
and suppresses the drive to sleep [38]. The intensity, spectra, duration, and timing of light
determine the magnitude and direction of phase shifting and potency of acute alerting
[39]. All wavelengths of light have a negative impact on sleep, but blue light elicits the
strongest effect due to the stimulation of intrinsically photosensitive retinal ganglion cells
[38]. Exposure to green light is capable of enhancing alertness and suppressing sleep [38],
while exposure to red light has the weakest effect on alertness and circadian phase shifting
[40]. Evidence from the laboratory, field and subject matter experts support the notion that
exposure to light during sleep episodes is disruptive to sleep quality and quantity [12-14,
29, 41-49]. Based on this evidence, all light should be eliminated from the sleep
environment. If indicator lights are necessary for identifying egress points, then they
should be dim and red [40].
There is strong evidence to suggest that individuals living in isolated and confined
environments away from typical solar light dark cues are prone to circadian desynchrony
due to self-selecting inappropriate patterns of light exposure [8, 50-54]. This circadian
misalignment leads to individuals experiencing a drive to sleep during scheduled wake
and an inability to sleep during scheduled sleep opportunities. In order to preserve a stable
24-hour pattern of work and sleep among the crewmembers, it may be desirable to
provide a strong cycling of light and darkness in common spaces to mimic the solar light
dark cycle and help crewmembers maintain a regular sleep-wake schedule and circadian
entrainment [55-56]. However, if such a strategy is utilized, it is important that
crewmembers maintain some autonomy in controlling dimmer, personal lighting as would
be the case at home on Earth. Similarly, crewmembers scheduled to be on night watch
may benefit from supplemental lighting in the vehicle command center in order to
enhance alertness and performance [57].
3.5 Noise
Noise is ever-present on space vehicles. We found that noise has been a major cause of
sleep disruption throughout the history of spaceflight [12, 19, 29, 58]. The current
guidelines allow for exposure to continuous noise above the WHO recommended
guidelines [33, 59]. In addition, the current NASA guidelines do not provide mitigations
against impulsive or intermittent noise [33]. We found that exposure to intermittent noise
is at least as disruptive to sleep as continuous noise exposure [11-12, 15, 19, 29, 58, 60-
61]. Given this evidence, exposure to noise be limited to below 35 dB, because exposure
to noise above this level is associated with a reduction in sleep quality and quantity, even
when individuals do not wake fully [59]. In addition, intermittent noise should be
minimized, so that it does not vary beyond 5 dB from background noise levels. There is
some evidence to suggest that exposure to continuous white noise less than 25 dB is
sufficient to mask intermittent noises [62], therefore it is desirable to allow crewmembers
access to white noise in their sleep chamber if desired. Earplugs and/or noise canceling
headphones should also be made available for crewmembers [63]. Due to crewmember
concerns about missing alarms while wearing earplugs, it may be desirable to develop
multi-sensory alarms that include auditory and visual stimulation [64-66].
3.6 Temperature and Humidity
The ambient temperature on early space vehicles varied widely. For optimal sleep, an
individual needs to reach his or her thermoneutral equilibrium and should have sufficient
bedding available to create a microclimate of between 25-35˚C (77-95˚F) [67-68]. Given
that there are wide individual differences in the optimal temperature for sleep, the sleep
environment on future space vehicles should be cool, but there should be sufficient
insulation available for crewmembers to modify their environment to suit individual
preferences [69-71]. This may mean providing crewmembers with sleeping bags of
different thicknesses, or a mechanism for layering sleeping bags together. It is also
desirable for sleeping bags to include vents to release heat, because the human core body
temperature falls and rises during a typical sleep episode [72]. Warming of proximal and
distal skin temperature has been associated with faster sleep onset [73-75] and
crewmembers have reported having difficulty sleeping due to cold feet and hands [19, 34],
therefore providing a way for crewmembers to warm their extremities prior to sleep may
be desirable.
The level of humidity in the environment can also influence sleep quality and quantity.
The optimal humidity range for human health is between 40-60% [19]. The presence of
humidity in the environment changes the perceived temperature. Higher humidity, with
high temperatures are disruptive to sleep [76]. Therefore, lower humidity of 50-60% is
optimal for sleep, particularly when ambient temperature is increased.
3.7 Air Quality
The optimal ambient gas mixture for sleep is equivalent to the air experienced at sea level
on Earth (78% nitrogen, 21% oxygen, 1% other gases) [16, 21, 77-86]. Similarly, the
optimal air pressure during sleep is equivalent to the pressure on the Earth at sea level [87-
88]. Air mixtures that deviate from these conditions, such as what mountaineers
experience during expeditions, results in disrupted sleep and periodic breathing [80, 82,
84, 88-90]. In depressurized environments, such as at elevation on Earth, supplemental
oxygen can reduce headaches, periodic breathing, and can improve sleep outcomes [91-
92]. Airflow is also associated with positive sleep outcomes and aids in the reduction of
O2 [85, 93] and intrusive odors, such as body odor, food, and mechanical smells [12, 34,
85]. Although there is little information on the impact of air pollution and particulates on
sleep quality and quantity, reports from lunar expeditions suggest that dust from planetary
extra vehicular activities may build up in the habitable environment [29, 34]. As a result,
the vents providing airflow to crew sleep chambers should include air filters to protect
against crewmembers breathing particulate matter and dust during sleep.
3.8 Involuntary Movement
Involuntary movement due to turbulence is associated with sleep disruption [94].
Therefore, vehicle movement and vibration should be minimized as much as possible.
Similarly, the microgravity environment results in the potential for crewmembers to free-
float during sleep episodes. Although some crewmembers have reported that they enjoyed
that experience, other crewmembers have reported that they prefer to be restrained while
sleeping [95]. Given that some individuals may not use harnesses and other attachments,
they should be designed, so that they can be removed or secured out of place when not in
use. Similarly, separate attachments should be available to secure the sleeping bag to the
wall of the sleep chamber if desired.
3.9 Summary
Although we present evidence to support the design of future space vehicles, it is possible
that new information will be revealed in the future. NASA supports a great deal of studies
in analog and spaceflight environments. As new information becomes available
recommendations may evolve and change. Such information should help to further define
the optimal sleep environment for deep space transit.
In summary, sleep is critical to crewmember health and performance. In order for
crewmembers to achieve optimal sleep, they must be provided with a sleep environment
that allows them to achieve quality sleep, free of external disruption. We found that the
optimal sleep environment is cool, dark, quiet, and is perceived as safe and private. There
are wide individual differences in the preferred sleep environment; therefore modifiable
sleeping compartments are necessary to ensure all crewmembers are able to select
personalized configurations for optimal sleep. A sub-optimal sleep environment is
tolerable for only a limited time, therefore individual sleeping quarters should be designed
for long duration missions. In a confined space, the sleep environment serves a dual
purpose as a place to sleep, but also as a place for storing personal items and as a place for
privacy during non-sleep times. This need for privacy during sleep and wake appears to be
critically important to the psychological well being of crewmembers on long duration
missions. Designing sleep chambers for optimal sleep health should produce benefits
beyond simply improving sleep quality and quantity on long duration missions.
References
1. Klerman, E.B., et al., Comparisons of the variability of three markers of the human circadian
pacemaker. J Biol Rhythms. 17(2), 181--193 (2002)
2. Akerstedt, T. and M. Gillberg, Effects of sleep deprivation on memory and sleep latencies in
connection with repeated awakenings from sleep. Psychophysiology. 16(1), 49--52 (1979)
3. Wilkinson, R., Some factors influencing the effect of environmental stressors upon
performance. Psychol Bull. 72(4), 260--272 (1969)
4. Dijk, D.J. and C.A. Czeisler, Paradoxical timing of the circadian rhythm of sleep propensity
serves to consolidate sleep and wakefulness in humans. Neurosci Lett. 166(1), pp. 63--68 (1994)
5. Dijk, D.J. and C.A. Czeisler, Contribution of the circadian pacemaker and the sleep homeostat
to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle
activity in humans. J Neurosci. 15(5 Pt 1), 3526--3538 (1995)
6. Compton, D.W. and C.D. Benson, Living and Working in Space: A History of Skylab. (1983)
7. Hoffman, S.J., Antarctic exploration parallels for future human planetary exploration: A
workshop report. National Aeronautics and Space Administration, (2002)
8. Basner, M., et al., Mars 520-d mission simulation reveals protracted crew hypokinesis and
alterations of sleep duration and timing. Proc Natl Acad Sci USA, 110(7), 2635--2640 (2013)
9. Yan, X.W. and M.E. England, Design evaluation of an arctic research station from a user
perspective. Environment and Behavior. 33(3), 449--470 (2001)
10. MSFC Skylab Structures, MSFC Skylab structures and mechanical systems mission evaluation,
In: NASA TMX-64824, June. National Aeronautics and Space Administration (1974)
11. International Federation of Air Line Pilots’ Associations, In-flight flight crew rest facilities.
(2013)
12. Hacker, B.C. and J.M. Grimwood, On the Shoulders of Titans: A History of Project Gemini.
NASA SP-4203. NASA Special Publication, 4203 (1977)
13. Caldwell, J.A., et al., Is fatigue a problem in army aviation: The results of a Survey of aviators
and aircrews, US Army Aeromedical Research Laboratory (2000)
14. Green, P.S. Special Report: Sleep Deprivation During Deployment and PTSD.
https://vanwinkles.com/for-our-returning-troops-post-traumatic-sleep-disorders-are-the-new-
ptsd, (2015)
15. Watt, C.G., Aircraft Fatigue Management, in Air War College. Air University (2009)
16. Stuster, J.W., Space station habitability: Recommendations based on a systematic comparative
analysis of analogous conditions. National Aeronautics and Space Administration, (1986)
17. Carrere, S. and G.W. Evans, Life in an Isolated and Confined Environment A Qualitative Study
of the Role of the Designed Environment. Environment and Behavior, 26(6), 707--741 (1994)
18. Hendrickx, B., The Kamanin Diaries 1969-1971. Journal-Britith Interplanetary Society,
55(9/10), 312--360 (2002)
19. Johnston, R.S., Skylab Medical Experiments Altitude Test (SMEAT) (1973)
20. Koros, A., C. Wheelwright, and S. Adam, An evaluation of noise and its effects on shuttle
crewmembers during STS-50/USML-1 (1993)
21. MacCallum, T.K. and J. Poynter, Factors affecting human performance in the isolated confined
environment of Biosphere 2, In: Third Annual Mid-Atlantic Human Factors Conference, 82--87
(1995)
22. Rosekind, M.R., et al., Crew Factors in Flight Operations XIII: A Survey of Fatigue Factors in
Corporate/Executive Aviation Operations, NASA: Moffett Field, CA (2000)
23. Vander Ark, S.T., A.W. Holland, and J. Wood, La Chalupa-30: Lessons Subsea Learned
Mission from a 30-day Analogue (1994)
24. Weiss, K., M. Feliot-Rippeault, and R. Gaud, Uses of places and setting preferences in a French
Antarctic station. Environment and Behavior, 39(2), 147--164 (2007)
25. Willshire, K.F., Human response to vibroacoustic environments of space vehicles. National
Aeronautics and Space Administration, Langley Research Center (1984)
26. Harrison, A.A., et al., Implications of privacy needs and interpersonal distancing mechanisms
for space station design (1988)
27. Yan, X.W., et al., A critical review of design and use of field tent shelters in polar regions. Polar
Record, 34(189), 113--122 (1998)
28. Legler, R.D. and F.V. Bennett, Space shuttle missions summary. National Aeronautics and
Space Administration: Mission Operations, Johnson Space Center (2011)
29. National Aeronautics and Space Administration, Apollo 11 Mission Report. Houston, TX
(1971)
30. National Aeronautics and Space Administration, Apollo 13 Mission Report. Houston, TX
(1970)
31. Shepard Jr, A., Apollo 14 mission report (1972)
32. Allen, C. and S. Denham, International Space Station Acoustics - A Status Report. In: 41st
International Conference on Environmental Systems (2011)
33. Broyan Jr., J.L., M.A. Borrego, and J.F. Bahr, International Space Station USOS Crew Quarters
Development. SAE International (2008)
34. Bluth, B. and M. Helppie, Soviet space stations as analogs (1986)
35. European Space Agency, Mars 500: Isolation Study (2010)
36. Czeisler, C.A. and J.J. Gooley, Sleep and circadian rhythms in humans. Cold Spring Harb Symp
Quant Biol. 72, 579--597 (2007)
37. Flynn-Evans, E.E., et al., Circadian misalignment affects sleep and medication use before and
during spaceflight. npj Microgravity. 2, 15019 (2016)
38. Lockley, S.W., et al., Short-wavelength sensitivity for the direct effects of light on alertness,
vigilance, and the waking electroencephalogram in humans. Sleep. 29(2), 161--168 (2006)
39. Lockley, S.W., Timed melatonin treatment for delayed sleep phase syndrome: the importance of
knowing circadian phase. Sleep. 28(10), 1214--1216 (2005)
40. Mien, I.H., et al., Effects of exposure to intermittent versus continuous red light on human
circadian rhythms, melatonin suppression, and pupillary constriction. PloS one. 9(5), e96532
(2014)
41. Bierman, A., M.G. Figueiro, and M.S. Rea, Measuring and predicting eyelid spectral
transmittance. Journal of Biomedical Optics. 16(6), 067011--067011 (2011)
42. Cho, J.R., et al., Let there be no light: the effect of bedside light on sleep quality and
background electroencephalographic rhythms. Sleep medicine. 14(12), 1422--1425 (2013)
43. Dijk, D.-J., et al., Sleep, circadian rhythms, and performance during space shuttle missions. The
Neurolab Spacelab Mission: Neuroscience Research in Space. 211--221 (2003)
44. Figueiro, M.G. and M.S. Rea, Short-wavelength light enhances cortisol awakening response in
sleep-restricted adolescents. International Journal of Endocrinology. 2012 (2012)
45. Figueiro, M.G., B. Plitnick, and M.S. Rea, Pulsing blue light through closed eyelids: Effects on
acute melatonin suppression and phase shifting of dim light melatonin onset. Nature and Science
of Sleep. 6, 149 (2014)
46. Grandner, M.A., et al., Short wavelength light administered just prior to waking: A pilot study.
Biological Rhythm Research. 44(1), 13--32 (2013)
47. Potter, J.J., et al., Polar field tent shelters and well-being of users. Environment and behavior.
30(3), 398--420 (1998)
48. Thompson, A., et al., Effects of dawn simulation on markers of sleep inertia and post-waking
performance in humans. Eur J Appl Physiol. 114(5), 1049--1056 (2014)
49. Zeitzer, J.M., et al., Millisecond flashes of light phase delay the human circadian clock during
sleep. J Biol Rhythms. 29(5), 370-376 (2014)
50. Arendt, J., Biological rhythms during residence in polar regions. Chronobiology International.
29(4), 379-394 (2012)
51. Halberg, F., et al., Human biological rhythms during and after several months of isolation
underground in natural caves. Bulletin of The National Speleological Society. 32, 89-115 (1970)
52. Kleitman, N., Sleep and Wakefulness. The American Journal of the Medical Sciences. 249(2),
140 (1965)
53. Miller, N.L. and J. Nguyen, Working the nightshift on the USS John C. Stennis: Implications for
enhancing warfighter effectiveness. In: Proceedings of the Human-Systems Integration
Symposium (2003)
54. Siffre, M., Biological rhythms, sleep, and wakefulness in prolonged confinement, in ESA,
Proceedings of the Colloquium on Space and Sea. 53--68 (1988)
55. Duplessis, C.A., et al., Submarine watch schedules: Underway evaluation of rotating
(contemporary) and compressed (alternative) schedules. Undersea & Hyperbaric Medicine. 34,
21--33 (2007)
56. Young, C.R., et al., At-sea trial of 24-h-based submarine watchstanding schedules with high and
low correlated color temperature light sources. Journal of Biological Rhythms. 30, 144--154
(2015)
57. Barger, L.K., et al., Sleep and cognitive function of crewmembers and mission controllers
working 24-h shifts during a simulated 105-day spaceflight mission. Acta Astronautica. 93, pp.
230--242 (2014)
58. Flynn-Evans, E.E., et al. Sleep duration, disruption and hypnotic use among 76 astronauts on
short duration missions. in NASA Human Research Program Meeting. Houston, TX (2012)
59. World Health Organization, Night noise guidelines for Europe (2009)
60. Fyhri, A. and G.M. Aasvang, Noise, sleep and poor health: Modeling the relationship between
road traffic noise and cardiovascular problems. Science of the Total Environment. 408, 4935--
4942 (2010)
61. Schmidt, F.P., et al., Effect of nighttime aircraft noise exposure on endothelial function and
stress hormone release in healthy adults. European Heart Journal. eht269 (2013)
62. Stanchina, M.L., et al., The influence of white noise on sleep in subjects exposed to ICU noise.
Sleep Med. 6(5), 423--428 (2005)
63. Department of the Army, Manual 6-22.5: Combat and Operational Stress Control Manual for
Leaders and Soldiers. Washington, DC: US Dept. of the Army. 111 (2009)
64. Kawada, T. and S. Suzuki, Change in rapid eye movement (REM) sleep in response to exposure
to all-night noise and transient noise. Archives of Environmental Health. 54, 336--340 (1999)
65. Muzet, A., Environmental noise, sleep and health. Sleep Medicine Reviews. 11, 135--142
(2007)
66. Rechtschaffen, A., P. Hauri, and M. Zeitlin, Auditory awakening thresholds in REM and NREM
sleep stages. Perceptual and Motor Skills. 22(3), 927-942 (1966)
67. Okamoto, K., et al., A survey of bedroom and bed climate of the elderly in a nursing home.
Applied Human Science. 17(3), 115--120 (1998)
68. Okamoto-Mizuno, K., K. Tsuzuki, and K. Mizuno, Effects of head cooling on human sleep
stages and body temperature. International Journal of Biometeorology. 48, 98--102 (2003)
69. Haskell, E.H., et al., The effects of high and low ambient temperatures on human slep stages.
Electroencephalography and Clinical Neurophysiology. 51, 494-501 (1981)
70. Lin, Z. and S. Deng, A study on the thermal comfort in sleeping environments in the subtropics
— Measuring the total insulation values for the bedding systems commonly used in the
subtropics. Building and Environment. 43, 905--916 (2008)
71. Häuplik-Meusburger, S., Architecture for Astronauts: An Activity-based Approach. Springer
Science & Business Media (2011)
72. Van Someren, E.J.W., et al., Circadian and age-related modulation of thermoreception and
temperature regulation: Mechanisms and functional implications. Ageing Research Reviews. 1,
721--778 (2002)
73. Kräuchi, K., et al., Warm feet promote the rapid onset of sleep. Nature. 401, 36--37 (1999)
74. Raymann, R.J.E.M., D.F. Swaab, and E.J.W. Van Someren, Cutaneous warming promotes sleep
onset. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.
288, R1589--R1597 (2005)
75. Raymann, R.J.E.M., D.F. Swaab, and E.J.W. Van Someren, Skin deep: Enhanced sleep depth by
cutaneous temperature manipulation. Brain. 131(2), 500--513 (2008)
76. Okamoto-Mizuno, K., et al., Effects of partial humid heat exposure during different segments of
sleep on human sleep stages and body temperature. Physiology & Behavior. 83, 759--765
(2005)
77. Daues, K.R., A history of spacecraft environmental control and life support systems (2006)
78. Gothe, B., et al., Effect of quiet sleep on resting and CO2-stimulated breathing in humans.
Journal of Applied Physiology. 50(4), 724--730 (1981)
79. Lo, Y.-L., et al., Genioglossal muscle response to CO2 stimulation during NREM sleep. Sleep.
29(4), 470 (2006)
80. Reite, M., et al., Sleep physiology at high altitude. Electroencephalography and Clinical
Neurophysiology. 38(5), 463--471 (1975)
81. Robin, E.D., et al., Alveolar gas tensions, pulmonary ventilation and blood pH during
physiologic sleep in normal subjects. Journal of Clinical Investigation, 37(7), 981 (1958)
82. Salvaggio, A., et al., Effects of high-altitude periodic breathing on sleep and arterial
oxyhaemoglobin saturation. European Respiratory Journal. 12(2), 408--413 (1998)
83. Schiffman, P., et al., Sleep deprivation decreases ventilatory response to CO2 but not load
compensation. CHEST Journal. 84(6), 695--698 (1983)
84. Selvamurthy, W., et al., Sleep patterns at an altitude of 3500 metres. International Journal of
Biometeorology. 30, 123--135 (1986)
85. Strøm-Tejsen, P., et al., The effects of bedroom air quality on sleep and next-day performance.
Indoor Air. (2015)
86. Szymusiak, R. and E. Satinoff, Maximal REM sleep time defines a narrower thermoneutral zone
than does minimal metabolic rate. Physiology & Behavior. 26, 687--690 (1981)
87. Miller, J.C. and S. Horvath, Sleep at altitude. Aviation, space, and environmental medicine.
48(7), 615--620 (1977)
88. Mizuno, K., K. Asano, and N. Okudaira, Sleep and respiration under acute hypobaric hypoxia.
The Japanese Journal of Physiology. 43, 161--175 (1993)
89. Dietz, T.E. An Altitude Tutorial. http://www.ismmed.org/np_altitude_tutorial.htm (2006)
90. Muza, S.R., C.S. Fulco, and A. Cymerman, Altitude Acclimatization Guide. Army Research Inst
of Environmental Medicine Natick MA Thermal and Mountain Medicine Division (2004)
91. Barash, I.A., et al., Nocturnal oxygen enrichment of room air at 3800 meter altitude improves
sleep architecture. High Altitude Medicine & Biology. 2(4), 525--533 (2001)
92. Luks, A.M., et al., Room oxygen enrichment improves sleep and subsequent day-time
performance at high altitude. Respiration physiology. 113(3), 247--258 (1998)
93. Federal Aviation Administration, Flightcrew Member Duty and Rest Requirements (2011)
94. Matsangas, P., N.L. Shattuck, and M.E. McCauley, Sleep duration in rough sea conditions.
Aerospace Medicine and Human Performance. 86, 901--906 (2015)
95. Petty, J.I. Space Shuttle Life, http://spaceflight.nasa.gov/history/shuttle-mir/history/h-b-
sslife.htm (2013)
