Sleep is vital to our health, including immunity from disease, heart function, and even reproductive vitality. A bad night’s sleep might contribute to poor performance at work and a sour emotional state the next day. It is difficult to assign a cost to a bad night’s rest, but I think most of us agree that it’s a cost we would rather avoid. Are there things we can change in our bedrooms that help ensure better, more restful, sleep? What about temperature, noise, or indoor air quality (IAQ)? Building science might save the day again!
Air temperature makes a difference. For most of us, the ideal temperature is 65 F degrees, or lower, in order to fall and stay asleep. It is better for us if the bedroom is cool rather than warm. This could be because our core temperature drops two or three degrees when we sleep; this does not happen easily in a bedroom that is too warm.
Most of us are awakened by loud noises that break a silence. Although you might debate with your sleeping mate about the usefulness of white noise*, many find it to be an effective way of masking disturbing sounds during sleeping hours. Dedicated white noise devices or phone apps can provide masking noise for a low cost. White noise can also be supplied as a side-effect from other devices, such as window air conditioners, window fans, or portable air cleaners.
Of the three factors discussed here that impact good sleep, IAQ is probably the most complex. Recently, I have come across a number of research articles that have examined the relationship between bedroom IAQ and ventilation, either provided by opening a window – natural ventilation – or by the operation of mechanical ventilation.
IAQ vs IEQ
IAQ is the well-known abbreviation for Indoor Air Quality; IEQ is a lesser-known abbreviation for Indoor Environmental Quality, with a significantly broader meaning. Whereas IAQ includes air quality only, IEQ includes, but is not limited to:
- Air quality
- Thermal comfort
- Lighting, both natural and artificial
- Moisture conditions (relative humidity)
- Interior aesthetics
- Spatial requirements
- Water quality
- Electromagnetic radiation
IEQ might include more or less than the elements on this list, depending on the indoor environment and the activities performed within it. The objective of good IEQ is to enhance the conditions and lives of the occupants from the perspectives of health, safety, performance, and wellbeing.
Finding the best sleep quality seems to be more than just IAQ; it includes a number of the IEQ elements, also.
As you might know, carbon dioxide, CO2, can be used as an indicator of the amount of fresh outdoor air entering a room. The average outdoor air levels of CO2 are around 400 parts per million (ppm). When a bedroom is occupied by two sleeping adults, they will consume oxygen and give off CO2. With no fresh outdoor air entering the room, the levels of CO2 will continue to climb, perhaps to unhealthy levels above 3000 ppm. On the other hand, if a window is open or a fan is bringing in 18 cubic feet per minute (cfm) of fresh outdoor air per person, the CO2 levels will remain under 1000 ppm; this is considered a healthy level. This amount of outdoor air supply is the equivalent of 2.25 ACH (Air Changes per Hour) for a 10’ x 12’ x 8’ bedroom.
Chandra Sekhar at the Building School of Design and Environment, National University of Singapore was the lead author of an important study** on this topic. His comprehensive review of bedroom ventilation, that includes a survey of 46 studies and 17 international residential dwelling ventilation standards, found that:
- The average CO2 concentrations in bedrooms ranged from 428 to 2585 ppm;
- The ACH in the bedrooms studied varied from 0.2 to 4.9 per hour;
- Sleeping with the bedroom door closed reduces air transference from the other parts of the house, increasing CO2 levels in the bedroom;
- CO2 levels during cold months were higher than when windows could be opened, indicating less outdoor air entering the bedrooms;
- Levels of CO2 were also higher during warmer months when the air conditioning (cooling) was operating. AC usually allows little or no fresh outdoor air to enter the rooms they serve.
Sekhar, et al, state: “. . . sleep quality will not be negatively affected when ventilation rates are such that CO2 levels remain below 750 ppm, while CO2 levels above 2600 ppm would disturb sleep quality and have a negative effect on the next-day cognitive performance.” High levels of CO2 can directly affect sleep quality. But because high levels of CO2 are an indication of lack of fresh outdoor air, other contaminants, such as formaldehyde, PM2.5, or nitrogen dioxide, might also be in the air in high concentrations; this adds to the negative impact.
At odds with Sekhar’s finding that sleeping with the bedroom door open lowers CO2 rates, is research from UL’s Firefighter Safety Research Institute demonstrating that closing the bedroom door helps prevent a fire from spreading, lowers smoke damage during a fire, and could even save lives. UL estimates that about 40% of people sleep with their bedroom doors closed. The organization promotes door closure with the phrase: “Close before you doze”.
Of the 17 ventilation standards from around the world examined by Sekhar and his team, none, including ASHRAE 62.2, specify minimum bedroom ventilation: “. . .it can be seen that there is no internationally accepted and standardized method of specifying prescriptive ventilation requirements in bedrooms. . .” (page 18) “. . .[this] absence in residential buildings is highlighted as a major limitation in standardization efforts and can be seen as an opportunity.” (page 18)
Some options that lead to better sleep can also be problematic: opening a window to allow more fresh outdoor air in and lower CO2 can lead to an increase in disturbing traffic noise. The white noise of an air conditioner can provide sleep-inducing temperatures, but often supplies little or no outdoor fresh air. Closing the bedroom door might decrease fire and smoke hazards and enhance privacy, but can limit air from other parts of the dwelling and lower ACH. I am sure you can add more “double-edged swords” to this list.
Setting up the ideal environment for a restful sleep is complex and will probably never be based on a formula that is good for all; sleepers have individual preferences. However, it is important to set a standard for a continuous flow of fresh outdoor ventilation air that will keep CO2 at a healthy level for the room where most of us spend one-third of our lives. Sweet dreams.
*White noise is defined as sound made up of all the frequencies the human ear is able to hear. “White” indicates a similarity to white light, which is made up of all the colors of the visible light spectrum. Because of the broad range of white noise frequencies, it is useful for masking other sounds that might be disturbing during sleep. Many people find electrically powered fans provide soothing white noise for sleep. A useful substitute for a box fan can be an air cleaner with a HEPA filter. This can supply a pleasant white noise while filtering harmful particles from the bedroom air.
** Chandra Sekhar, Mizuho Akimoto, Xiaojun Fan, Mariya Bivolarova, Chenxi Liao, Li Lan, Pawel Wargocki. “Bedroom ventilation: Review of existing evidence and current standards”. Journal of Building and Environment, Volume 184, 15 October 2020.