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

It feels like you've gotten enough sleep, but you still wake up feeling rested?

You might be surprised at the impact the air quality in your bedroom can have on your sleep and how you feel the next day.

Sleep is a fundamental need that restores the body and mind, relieves fatigue, restores energy levels, and ensures optimal brain function.

Several studies show that people spend about a third of their lives sleeping. There are four stages of sleep, divided into two types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

NREM sleep has three stages:

• NREM Stage 1: This is the transition period between wakefulness and sleep. It lasts about 5 to 10 minutes.

• NREM Stage 2: In this stage, 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 phases.

• NREM Stage 3: Also known as deep or slow-wave sleep, this is the most restorative stage. During this stage, muscles relax, and blood pressure and respiratory rate decrease. This is the stage in which we sleep most deeply.

REM (rapid eye movement) sleep: This is the stage in which dreams occur. The brain is more active, and the body is relaxed and still. The eyes move rapidly.

The complete sleep cycle lasts about 90 minutes and repeats four to six times per night. Different factors can influence each sleep phase. Any disruption to one of these phases can lead to poor sleep quality.

Indoor environmental quality in the bedroom

A bedroom's indoor environmental quality (IEQ) significantly influences 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 European Federation of Heating, Ventilation and Air Conditioning Associations (REHVA) indicates that a carbon dioxide (CO₂) concentration of less than 800 ppm is considered ideal, with 1,000 ppm representing the upper acceptable limit. Dust, allergens, and pollutants (PM2.5 and PM10) should also be minimized.

Thermal comfort: The National Sleep Foundation (NSF) suggests that the ideal sleeping temperature varies slightly from person to person; however, experts generally consider temperatures between 15°C and 20°C to be the most suitable for quality sleep. Caddick et al. (2018) conducted a review to identify ideal environmental conditions for sleep, highlighting that relative humidity should be between 40% and 60% for an ideal sleep environment.

Noise and acoustic levels: WHO guidelines for noise in residential environments indicate that the main effects of noise are sleep disturbance, annoyance, and speech interference. To prevent sleep disturbances, the maximum noise limits in the bedroom are 30 dB LAeq¹ for continuous noise and 45 dB LAmax² for isolated noise.

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

Sleep quality can be influenced by several factors, one of the most important being the bedroom's indoor environmental quality (Liao, C. 2022). Although we spend about a third of our lives sleeping, bedroom air quality varies greatly. This variation results from the delicate balance between thermal comfort, energy efficiency, and ventilation.

Among the IEQ parameters, CO₂ concentration has a significant impact on sleep. This increases dramatically in unventilated rooms. The main source of CO₂ in the bedroom is the occupants' own breathing during sleep. An experiment conducted at the University of Cassino and Southern Lazio demonstrated that there is a drastic increase in CO₂ concentration in unventilated rooms. The graph below shows the CO₂ concentration in rooms occupied by one or two people. In both cases, CO₂ levels are elevated due to insufficient ventilation. Self-reported sleep quality assessments show that those exposed to the CO₂ concentrations indicated in the graph experienced poor sleep quality that night.

Fig. Carbon dioxide concentration in rooms with one and two people

A pilot study (Buonanno et al., 2024) on the effects of environmental parameters on sleep and respiratory parameters in individuals with high-quality sleep demonstrated that a 100 ppm increase in CO₂ concentration is associated with an approximately 0.29% reduction in sleep quality . In that study, a 2,697 ppm difference from the median CO₂ value resulted in a 4.5% reduction in deep sleep (slow-wave sleep) . According to the ASHRAE report on ventilation, IEQ, and sleep quality, several studies suggest that sleep quality is not affected when bedroom ventilation keeps CO₂ levels below 750 ppm . However, when these levels rise to approximately 1,150 ppm , sleep quality can be impaired, and concentrations above 2,600 ppm can further affect next-day cognitive performance.

Natural ventilation of the bedroom before bed can reduce the initial CO₂ concentration, but in a closed indoor environment, it will increase again during the night. In winter, low temperatures and concerns about energy conservation make natural ventilation less viable. Keeping the bedroom door open at night can help with air circulation, distributing CO₂ to other rooms in the house. Additionally, factors such as the presence of pets and smoking in the bedroom contribute to high CO₂ levels, which can further impair sleep quality.

Buonanno et al.'s (2024) findings on the impact of relative humidity (RH) on sleep quality indicate that a 1% increase in RH is associated with a 0.1% decrease in sleep quality . Although several studies have analyzed the effects of temperature and relative humidity on sleep separately, their impact on sleep appears to be minimal. However, when combined , these factors have a more pronounced effect, leading to a decrease in overall sleep quality. This suggests that the interaction between temperature and humidity plays a key role in maintaining an optimal sleep environment.

In short, poor bedroom air quality affects several sleep phases . Poor sleep also impacts academic and professional performance the next day.

Conclusion

Sleep quality is closely linked to bedroom environmental conditions. Key factors such as carbon dioxide concentration, temperature, humidity, noise, and ventilation influence sleep efficiency and next-day performance. Elevated CO₂ levels, often caused by poor ventilation, are consistently associated with reduced sleep quality and lower next-day cognitive performance. While each environmental factor has a moderate effect when considered individually, their combined impact is substantial. Therefore, maintaining good indoor air quality , especially through adequate ventilation and optimal thermal comfort, is crucial for restful sleep and optimal daytime functioning.

References:

1. Equivalent sound level representing the sound pressure level that would be produced by a constant noise with the same sound energy.

2. The maximum sound level recorded during a measurement period.

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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 . Construction 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. Construction 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 required 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 noise in residential environments: https://www.who.int/europe/news-room/fact-sheets/item/noise