Designing bedrooms in cities for a better night's sleep

We need to start building apartments for a breath of fresh air

There is a ferocious debate about laws that mandate windows in apartment bedrooms. But what if we’re focusing on the wrong thing? The evidence suggests that fresh air is far more important than light for a good night’s sleep.

Maintaining clean air in the bedroom is more difficult than it sounds. When you exhale you breathe out carbon dioxide. Even with a HEPA filter CO2 can build up, reducing the quality of your sleep and making it more difficult to focus the next day.

The benefits of natural light in the morning are far less well established. Many people find that a sunrise helps them wake up, but there’s no evidence natural light is any better than a sun-rise lamp paired with plenty of bright natural light throughout the day.

Opening windows can release CO2, and planning regulations for apartments often require windows in bedrooms to let fresh air in. But most apartments in cities are on noisy and polluted streets. It’s very difficult to maintain a quiet and dark environment while keeping the bedroom windows open to let in fresh air. And as air conditioning becomes more common, opening windows also results in losing precious cool air.

Good urban planning can reduce street noise, but even open windows on quiet streets let a city’s light pollution in. We need to accept that most apartment dwellers are better off leaving their blinds down with their windows closed at night.

Closing windows ensures the bedroom stays cool, dark, and quiet, but how do you get fresh air in? The best solution is mechanical ventilation. Heat transfer units (aka energy recovery ventilators) swap dirty inside air for filtered outside air, recovering the heat along the way so that the inside stays the right temperature.

Once the purview of wealthy eccentrics on Grand Designs, these machines are now reasonably affordable if they’re installed when an apartment is built. Mechanical ventilation is the only real way to ensure a good night’s sleep in a small bedroom on a busy street.

In a world of housing abundance, nobody would build apartments without bedroom windows because they would never sell. But given zoning restrictions artificially raise rents, allowing well-ventilated window-less bedrooms is one way to even out the score.

Why are windows in bedrooms good?

Public health was the main reason that window mandates were passed in many Western countries. Wooden apartments without sprinklers, polluting gas stoves, and tuberculosis risk made the public health case for windows inescapable.

But things are not as bad as they were in the tenement era. We now have electric stoves and HEPA filters that can filter everything except CO2, making indoor air quality better than even outdoor air. And deadly fires are much less common than they once were, with sprinklers largely mitigating the risk of not having easy egress through a bedroom.

What’s left now are two main benefits to windows in modem apartments:

• They can be opened to reduce levels of CO2, improving sleep quality; and

• They provide natural light in the morning that may improve your circadian rhythm.

Both of these are great benefits to having a window in the bedroom, but the health advantages they provide are not as big as you might think.

Clean air in the bedroom can improve sleep quality and health

High CO2 can make a room feel stuffy, and it has been shown to reduce the quality of sleep - even in the absence of other pollutants.

There have been numerous studies that have assessed sleep quality at varying CO2 levels. They usually involve a small number of participants. Some have been purely observational while others have involved fans that pump out CO2 when it is too high. One study even randomized participants to open v close windows overnight.

The best estimate I have from one Australian study is that for every 100ppm increase in CO2 concentration, deep sleep falls by 4%. Sleep quality is unaffected at CO2 levels below 700, steadily rising from 700 and above. At levels higher than 2,600, cognitive performance has been shown to decrease the next day.

I have a CO2 monitor in my bedroom, and if I leave the doors and windows closed CO2 rises to levels that will definitely affect my sleep:

Mechanical ventilation is better than windows because people need to close their windows overnight

Windows might be great for reducing CO2, but they won’t help if you leave them closed.

In the anglosphere’s post-war suburban ideal, most people lived in a moderate climate on quiet streets with leaky houses and cross-ventilated windows that could stay open all night even during the warmer months. But for new apartment dwellers, energy efficiency standards require homes to be airtight. Cool air also improves sleep, and people rely on air-conditioning to maintain an even temperature. Opening the bedroom window means letting the cool air escape and exposing yourself to noisy and polluting traffic outside.

It’s for this reason that I doubt regulations mandating windows will change indoor CO2 quality. There should instead be much wider installation of energy recovery ventilators (ERVs) units during building construction. These devices have fans that swap dirty indoor air with filtered outdoor air while keeping the nice temperature that the indoor air already has.

This heat exchanger is $450USD on Amazon

ERVs bring in fresh air from outside, in contrast to heat pumps or reverse cycle air conditioners that tend to re-circulate existing air. ERVs are difficult to retro-fit in existing apartments, making it essential that they are considered when new apartment buildings are being designed.

Light improves circadian rhythm

Natural light is clearly good for human health, it’s just not clear how much having a natural sunrise improves your day.

Light when you’re trying to get to sleep is bad. If you put people in a lab and give them bright light in the evening their melatonin will be suppressed, which makes it much harder to sleep at the right time. Even light from 1-5am increases cortisol levels, presumably reducing sleep quality. Real-world studies have found that people who sleep in bright bedrooms at night toss and turn in bed and report poorer quality sleep.

But it’s not ideal to live in pitch black all the time. If you deprive people of all light, they tend to go to bed later and later each night, until their sleep cycle is completely reversed. Daylight suppresses your melatonin, helping you maintain a 24-hour cycle.

Adapted from Figure 3 in this study. Light from 7-9am that shifts your sleep cycle by 1 hour is helpful. I’ve said “melatonin” here but alpha amlylase, cortisol and amylase are also involved.

Morning light also seems to help. When researchers woke up lab participants at 6am with bright lights they got up early again the next morning even without the stimulus. And when people stuck in an Antarctic winter with little daylight were exposed to bright light in the morning it shifted their sleep cycle, helping them to rise and improving their cognitive function. But in both studies the morning light didn’t help people get to bed earlier, so total sleep time was reduced - at least in the short term.

But natural light from bedroom windows probably isn’t that important

So how much light do you need in the morning? And at what time?

This study found that even a bedside dawn simulator can be effective at improving self-assessed sleep quality. Another study found that 10 lux (a bright light bulb) was enough to improve your circadian rhythm in a lab setting.

It’s also not at all clear that you need bright light before you wake up. Getting bright light only after waking was enough to raise cortisol levels in this study, meaning that getting out of bed and enjoying some living room sun could have real benefits. Office workers are more depressed if they don’t have natural light throughout the day, and so the Munger dream of a completely dark living environment is probably not a good idea.

All of these studies are very short-term. What you won’t find above is evidence that natural light in the bedroom is good for your long-term health. If you personally find a sunrise makes it easier for you to get out of bed then it’s worth it for you. If you like to keep the blinds closed that’s fine too.

Should we mandate windows in bedrooms?

Windows in bedrooms do have some benefits, but they also have real costs. If you require windows, fewer apartments can be built on a site. When that site is developed, fewer people will get homes. There will be a longer queue at open for inspections, and rents will rise.

Regulating for windows in bedrooms can be especially troublesome because some windowless bedrooms still let in lots of light. “Borrowed light” is where windows allow light through a living room to the outside. They provide enough light so that you know what time of day it is, but not a bright sunrise.

This kind of set up can work really well for one-bedroom apartments where privacy between the living room and the bedroom doesn’t really matter.

Requiring good ventilation in bedrooms on the other hand will affect rents much less. Because these regulations increase construction costs without decreasing the amount of housing that can be built, the effect on housing supply for in-demand areas will be much less, and more of the cost will be paid for by owners of land.

What this means for you

For most existing apartments, mechanical ventilation is currently too difficult to install. And the technology is not quite there yet to provide cheap window-mounted ventilation units for renters. So what can you do to improve your sleep?

Leaving your window open all night might still be the best option, but you might also consider briefly opening the window before bed to lower CO2 levels before you sleep (The Germans call this “Lüften”). If you live alone you can also leave the door to the living room open to expel some of your overnight CO2 into that larger space.

Try your best to make the room quiet and pitch-black at night. If you can time your blinds to open around sunrise that’s a bonus, and if not a bright sunrise lamp will probably have a similar effect. In summer, keeping the bedroom cool can also help to improve sleep quality.

Home CO2 monitors are becoming much more common, so if you have any overnight readings post them below with details of your room!