It is recommended that adults sleep 7-9 hours each night [1]. However, ensuring sufficient time in bed (e.g., >9 hours) may not be enough to meet this recommendation. People often experience difficulty falling asleep (e.g., >30mins) and/or staying asleep (wake after sleep onset >20mins) [2]. Sleep difficulties may be multifactorial, and the sleep environment may be one of those factors [3]. It is essential to understand that the human brain and body interact with the environment not only when we are awake but also during the transition from wake to sleep (falling asleep) and whilst asleep. Therefore, the sleep environment influences (positively/negatively) our sleep. Specifically, an inappropriate sleep environment reduces the duration and quality of sleep. An improved sleep environment, however, facilitates falling asleep and maintaining sleep. Several sleep environmental factors have been discussed in the scientific literature. In particular, temperature, noise, light, bedding, and air quality influence our sleep [4, 5]. Let’s look at each of these in more depth:

Temperature

Our core body temperature follows a circadian rhythm (~24 hrs). In particular, our core body temperature starts to decline just before we fall asleep and reaches its lowest (nadir) around 6 hours after sleep onset before it increases again [6]. This circadian rhythm is an important factor in falling asleep, staying asleep, and waking up naturally [7-9]. An inappropriate thermal environment (e.g., too hot or too cold) affects our core body temperature,  resulting in trouble falling asleep or waking up in the middle of the night.

Studies have shown that sleeping in a room with high air temperature (e.g., 29°C) increases the time to fall asleep and the number/duration of nocturnal awakenings, reducing total sleep time [3]. On the other hand, studies have shown that the time awake after sleep onset increases when the sleep environment is cold (e.g., 13°C). However, this negative effect was absent when sufficient insulation (e.g., blankets) was provided [10].

It is recommended to keep the sleep environment cool. Importantly, whilst the temperature is a crucial component, other thermal factors influence sleep, including humidity, airflow, and the microclimate created by the mattress, cover, and clothing.

Noise

Noise can not only interfere with falling asleep but also affect once asleep. When we sleep, our brain filters/blocks out noise. However, exposure to noise during sleep can still have adverse effects on our sleep, including nocturnal awakenings and changes in sleep architecture [11]. Whether or not a specific noise disrupts sleep depends on various factors, including noise characteristics (e.g., volume, frequency), interaction with other noises (e.g., white noise), as well as how deep we are asleep (e.g., stage 1) [5, 11]. Interestingly, intermittent noise (e.g., traffic) has been shown to be more sleep-disruptive than continuous noise (e.g., ventilation) [12].

It is recommended to have a quiet sleep environment. Try to avoid any intermittent noises. Continuous noise (e.g., white noise) may somewhat mask intermittent noise without interfering with sleep.

Light

Our biological clock is a vital sleep regulator. This circadian rhythm is synchronised with our planet’s light/dark rhythm [13]. With the invention of artificial light, humans are often exposed to light at night. Although our eyelids act as a light blocker, some light still passes through, which may negatively affect our sleep [4].

A study has shown that having a bedside light on (~40 lx) increases nocturnal awakenings. Low light levels (e.g.,10 lx), such as street lights from outside entering the room, have also been shown to disrupt sleep [14].

Therefore, the sleep environment should be dark. Avoid light entering your room from outside, and turn off all light sources inside your room.

Bedding

Bedding includes the mattress, pillow, and cover. Individual products vary in characteristics, such as material, firmness, and thickness, which impact physical and thermal comfort. An inappropriate mattress and pillow (e.g., too firm, high heat retention) have been shown to result in physical discomfort (e.g., muscular stiffness) and sweating, resulting in nightly awakenings [15]. The scientific literature on optimal bedding is limited. The right choice of bedding is individual and depends on multiple factors, such as body height and weight, sleeping position, having (or not having) a bed partner, and thermoregulation (e.g., running hot at night). Weighted blankets have been proposed to improve sleep. However, clear evidence is still lacking [16].

Air quality

A lack of ventilation in the sleep environment results in carbon dioxide (CO2) accumulation whilst asleep, often perceived as stuffy air in the morning[17]. Elevated CO2 levels have been shown to reduce sleep quality [3]. Other gaseous components (e.g., particulate matter) and scents (e.g., smoke) from within or by entering the room from outside have also been shown to be detrimental to sleep [3]. Sleeping at altitude, when not acclimatised, has also been shown to reduce sleep quality [18, 19].

It is recommended to ensure sufficient ventilation within the sleep environment. Furthermore, keeping the bedroom clean and regularly changing bed sheets improve the air quality in the sleep environment.

RECOMMENDATIONS/APPLICATIONS

When considering the sleep environment, the following steps should be taken.

1. Identification: Identify the source of the sleep-disturbing environmental factor. For instance, do you wake up too early because sunlight enters your room?
2. Elimination: Eliminate the identified environmental factor. For instance, install block-out blinds to avoid light entering your room.
3. Reduction: If you cannot eliminate an environmental factor, reduce its impact on your sleep. For instance, wear an eye mask to avoid the adverse effect of light.
4. Evaluation: Importantly, by eliminating one environmental factor or reducing its impact on your sleep, the effect of another environmental factor may increase. For instance, wearing an eye mask reduces the impact of light. However, it can be uncomfortable (e.g., itchy, too hot) and disturb sleep. It is therefore recommended to evaluate other environmental factors when a specific factor has been eliminated or its impact reduced.

CONCLUSION

The sleep environment is a crucial factor for getting sufficient sleep of good quality and warrants consideration. In particular, research has shown that temperature, noise, light, bedding, and air quality are critical environmental factors. An identified sleep-disturbing environmental factor should be eliminated where possible. If its elimination is not possible, reduce the factor’s impact on your sleep and evaluate other environmental factors. To date, sleep environment research is scarce, allowing only limited evidence-based recommendations.

Are you interested in how the sleep environment affects human sleep? We are currently conducting research on shift workers (e.g., FIFO, DIDO) in the mining, oil and gas, harvesting, and quarrying industries in Australia, investigating the relationship between their sleep environment and their sleep. Find more information and access the survey at the Sleep Room (www.sleeproom.au).

REFERENCES

1. Hirshkowitz, M., et al., National Sleep Foundation’s updated sleep duration recommendations. Sleep health, 2015. 1(4): p. 233-243.
2. Ohayon, M., et al., National Sleep Foundation’s sleep quality recommendations: first report. Sleep health, 2017. 3(1): p. 6-19.
3. Caddick, Z.A., et al., A review of the environmental parameters necessary for an optimal sleep environment. Building and environment, 2018. 132: p. 11-20.
4. Ohayon, M.M. and C. Milesi, Artificial outdoor nighttime lights associate with altered sleep behavior in the American general population. Sleep, 2016. 39(6): p. 1311-1320.
5. Zhang, N., B. Cao, and Y. Zhu, Indoor environment and sleep quality: A research based on online survey and field study. Building and Environment, 2018. 137: p. 198-207.
6. Refinetti, R., The circadian rhythm of body temperature. Front Biosci, 2010. 15(1): p. 564-594.
7. Gilbert, S.S., et al., Thermoregulation as a sleep signalling system. Sleep medicine reviews, 2004. 8(2): p. 81-93.
8. Krauchi, K. and T. Deboer, The interrelationship between sleep regulation and thermoregulation. Frontiers in Bioscience-Landmark, 2010. 15(2): p. 604-625.
9. VanSomeren, E.J., More than a marker: interaction between the circadian regulation of temperature and sleep, age-related changes, and treatment possibilities. Chronobiology international, 2000. 17(3): p. 313-354.
10. Tsuzuki, K., K. Okamoto-Mizuno, and K. Mizuno, The effects of low air temperatures on thermoregulation and sleep of young men while sleeping using bedding. Buildings, 2018. 8(6): p. 76.
11. Pirrera, S., E. De Valck, and R. Cluydts, Nocturnal road traffic noise: A review on its assessment and consequences on sleep and health. Environment international, 2010. 36(5): p. 492-498.
12. Basner, M., et al., Auditory and non-auditory effects of noise on health. The lancet, 2014. 383(9925): p. 1325-1332.
13. Roenneberg, T., et al., Light and the human circadian clock. Circadian clocks, 2013: p. 311-331.
14. Cho, J.R., et al., Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms. Sleep medicine, 2013. 14(12): p. 1422-1425.
15. Radwan, A., et al., Effect of different pillow designs on promoting sleep comfort, quality, & spinal alignment: A systematic review. European Journal of Integrative Medicine, 2021. 42: p. 101269.
16. Eron, K., et al., Weighted blanket use: a systematic review. The American Journal of Occupational Therapy, 2020. 74(2): p. 7402205010p1-7402205010p14.
17. Ramos, J., et al., Influence of indoor air quality on sleep quality of university students in Lisbon. Atmospheric Pollution Research, 2022. 13(2): p. 101301.
18. Weil, J.V., Sleep at high altitude. High altitude medicine & biology, 2004. 5(2): p. 180-189.
19. Ainslie, P.N., S.J. Lucas, and K.R. Burgess, Breathing and sleep at high altitude. Respiratory physiology & neurobiology, 2013. 188(3): p. 233-256.