Do you often feel drowsy in the morning, even after a whole night’s sleep? 

Does it seem like you slept enough but you still wake up not feeling rested? 

You may be surprised how much your bedroom’s air quality can affect your sleep and how you feel the next day.

Sleep is a fundamental need that restores the body and mind, alleviating fatigue, replenishing energy levels, and ensuring optimal brain function. Several studies show that people spend approximately one-third of their lives sleeping. There are four stages of sleep, divided into two types: non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). 

NREM has three stages:

NREM stage 1: this is the transition period between wakefulness and sleep. It lasts around 5 to 10 minutes. 

NREM stage 2: in this stage, the body temperature drops, and the heart rate begins to slow. The brain begins to produce sleep spindles. It lasts approximately 20 minutes. NREM stages 1 and 2 are light sleep stages.

NREM stage 3: this stage is also known as deep or slow wave sleep. This is the most restorative phase. During this stage, the muscles relax, and blood pressure and breathing rate drop. We sleep deepest in this stage.

Rapid eye movement (REM) sleep: during this stage, dreams occur. The brain becomes more active, and the body becomes relaxed and immobilised. Our eyes move rapidly. 

The complete sleep cycle lasts around 90 minutes and is repeated four to six times per night. Different factors can influence each sleep stage. Distractions in any one of these sleep stages will cause us to experience poor sleep quality.

Bedroom indoor environmental quality

The indoor environmental quality (IEQ) in a bedroom significantly affects sleep quality, health, and overall well-being. It encompasses factors such as air quality, ventilation, lighting, temperature, humidity, and noise levels.

A study by the Federation of European Heating, Ventilation, and Air Conditioning Associations (REHVA) indicates that an indoor carbon dioxide concentration below 800 ppm is considered optimal, while 1,000 ppm represents the upper limit of the acceptable concentration level. Dust, allergens, and pollutants (PM2.5 & PM10) should also be minimised.

Thermal comfort: the National Sleep Foundation (NSF) suggests that optimal sleeping temperature is slightly different for everyone; nonetheless, in general, experts consider temperatures >15°C and <20°C to be optimal for high-quality sleep. Caddick et al. (2018) conducted a review to identify the optimal environmental conditions for sleep quality, emphasising that relative humidity should be maintained between 40-60% for an ideal sleeping environment.

Acoustic & noise levels: WHO guidelines for community noise state that in dwellings, the critical effects of noise are sleep, annoyance, and speech interference. To avoid sleep disturbance, the upper limits of noise levels in the bedroom are 30 dB LAeq¹ for continuous noise and 45 dB LAmax² for single sound events.

The impact of bedroom air quality on sleep and next-day performance

Sleep quality can be influenced by different factors, one of the most significant being the bedroom’s indoor environmental quality (Liao, C.2022). Even though people spend roughly one-third of their lives sleeping, the air quality in bedrooms fluctuates significantly. This variability is due to the intricate equilibrium between factors such as thermal comfort, energy efficiency, and ventilation. 

Among the IEQ parameters, CO₂ concentration has a significant impact on sleep. It increases dramatically in bedrooms with no ventilation. The main source of carbon dioxide in the bedroom is the occupants’ own breathing during sleep. An experiment conducted at the University of Cassino and Southern Lazio to assess indoor environmental quality in bedrooms showed that there is a dramatic increase in the CO₂ concentration in bedrooms without ventilation. The figure below illustrates the CO₂ concentration in double and single-occupied bedrooms. In both cases, the concentration of CO₂ is high due to a lack of sufficient ventilation. The self-reported sleep quality assessment indicates that participants exposed to the CO₂ concentrations in their bedrooms, as presented in the graph below, experienced poor sleep quality on the corresponding night.

Fig. Carbon dioxide concentration in double and single occupied bedrooms

A pilot study (Buonanno et al., 2024) on the effects of environmental parameters in bedrooms on sleep and respiratory parameters for individuals with high-quality sleep showed that an increase of 100 ppm in CO₂ concentration levels correlates to an approximate 0.29% decline in sleep quality. In this study, a difference of 2697 ppm in CO₂ concentration from the median value resulted in a 4.5% reduction in the slow wave sleep stage. According to the ASHRAE issue brief on ventilation, IEQ, and sleep quality in bedrooms, several studies suggest that sleep quality remains unaffected when bedroom ventilation keeps CO₂ levels below 750 ppm. However, when CO₂ levels rise to around 1,150 ppm, sleep quality may be disrupted, and concentrations exceeding 2,600 ppm can further impair cognitive performance the following day.

Natural ventilation in the bedroom before bedtime primarily lowers the initial CO₂ concentration; however, in a constant indoor environment, the concentration will continue to rise throughout the night. During the winter season, low temperatures and energy saving make natural ventilation less practical. Keeping the bedroom door open overnight can facilitate air circulation, thereby distributing CO₂ through the indoor spaces. Additionally, factors such as the presence of pets and smoking in the bedroom contribute to high CO₂ levels, which can negatively impact sleep quality.

The findings of Buonanno et al. (2024) on the impact of relative humidity (RH) on sleep quality indicate that a 1% increase in RH is associated with a 0.1% reduction in sleep quality. While various studies have examined the individual effects of temperature and relative humidity on sleep, their influence appears to be minimal when considered independently. However, when combined, these factors have a more pronounced effect, leading to a decline in overall sleep quality. This suggests that the interplay between temperature and humidity plays a critical role in maintaining an optimal sleep environment.

Overall, poor indoor air quality in the bedroom affects different sleep stages. Poor sleep quality then affects the next-day class and work performance.

Conclusion

Sleep quality is closely linked to the indoor environmental conditions in the bedroom. Key factors such as carbon dioxide concentration, temperature, humidity, noise, and ventilation all impact sleep efficiency and next-day performance. High CO₂ levels often resulting from poor ventilation are consistently associated with reduced sleep quality and diminished cognitive performance the following day. While each environmental factor may have a mild effect independently, their combined impact is substantial. Therefore, maintaining good indoor air quality, especially through adequate ventilation and optimal thermal comfort, is essential for restful sleep and optimal daytime functioning.

References:

¹. Equivalent sound level that represents the sound pressure level that would be produced by a constant noise level with    the same amount of noise energy.
². The maximum sound level reached during a measurement period.

Bibliography:

• Liao, C., Fan, X., Bivolarova, M., Laverge, J., Sekhar, C., Akimoto, M., Mainka, A., Lan, L., & Wargocki, P. (2022). A cross-sectional field study of bedroom ventilation and sleep quality in Denmark during the heating season. Building and Environment, 224, 109557. https://doi.org/10.1016/j.buildenv.2022.109557
• Buonanno, G., Canale, L., Solomon, M., Smith, M., & Stabile, L. (2024). Effect of bedroom environment on sleep and physiological parameters for individuals with good sleep quality: A pilot study. Building and Environment, 265, 111994. https://doi.org/10.1016/j.buildenv.2024.111994
• Caddick, Zachary A., Kevin Gregory, Lucia Arsintescu, Erin E. Flynn-Evans, A review of the environmental parameters necessary for an optimal sleep environment, Building and Environment, Volume 132, 2018, Pages 11-20. https://doi.org/10.1016/j.buildenv.2018.01.020
• WHO guidelines for community noise. https://www.who.int/europe/news-room/fact-sheets/item/noise