How to run a fair three-night bedroom air check

A fair check measures your bedroom on ordinary nights, set up the same way each time, for at least three nights. That turns one night's clue into a pattern you can actually rely on.

Live as usual

The goal is to measure the room the way you really use it. If you usually sleep with the door shut and the window closed, keep it that way. Opening things up because you're measuring would give you a good number for a room you don't actually sleep in.

There will be time to experiment later. First you need an honest starting point.

Keep the setup the same each night

Try to keep these the same on every measuring night:

• The people (and pets) sleeping in the room.
• The door: closed, ajar or open.
• The window: closed or open, and by how much.
• Fans, any ventilation system, and heating or cooling settings.
• The monitor's spot.

Each of these can move the result. ASHRAE notes that how much CO₂ a person breathes out depends on things like age, body size and activity, so an extra sleeper changes how fast CO₂ builds. Doors and vents matter too. In one study of new homes in Scotland, bedrooms with closed windows averaged about 2,000 ppm overnight when the door and window-frame vents were shut, and about 1,000 ppm when both were open. A door that's ajar one night and shut the next can easily explain a big difference.

Note anything different

The UK Health and Safety Executive (HSE) recommends recording CO₂ readings along with the number of people present and the type of ventilation in use. At home, that can be one short line each morning:

• "Guest slept in here."
• "Window cracked after midnight."
• "Door open from 3 am."
• "Heat ran most of the night."

The app asks for this note each morning, but a scrap of paper works just as well. Notes are what make an unusual night explainable instead of confusing.

Why three nights

One night is a clue. A pattern across nights is an answer. HSE cautions that single or "snapshot" readings can be misleading, and one night is a kind of snapshot.

You can measure for anywhere from two to seven nights. Three is a good balance: enough to see whether the room behaves the same way, and short enough to fit into a normal week. If your routine changes a lot between weeknights and weekends, a few extra nights will give a fuller picture.

Your typical night, and the goal

Each night gets one number. When your monitor records the whole night, it's the average CO₂ while you were asleep, weighted by time, which is the measure the sleep research uses. When you type a number in, it's the overnight high, which is always a little above the average. Across nights, the middle value is your typical night.

The goal for a bedroom is 800 ppm or less (why 800). For example, nights averaging 1,050, 1,120 and 1,090 ppm give a typical night of 1,090: Stuffy, and 290 ppm above the goal. Those three nights sit close together, so the result is consistent.

When nights disagree

If your highest and lowest nights are 400 ppm or more apart, the report marks the check as variable. That's a prompt to look at your notes, not a sign that something went wrong.

For example, nights of 1,050, 1,100 and 1,620 ppm have a spread of 570. If the notes show a guest slept in the room on the 1,620 night, that explains it, and the other two nights describe your usual setup. If nothing was different, a few more nights will help show which result is typical.

A variable result can still be useful. It shows that the room responds to changes, which is the idea behind trying one free change at a time.

What the report can and can't conclude

A fair three-night check can tell you:

• Whether the bedroom typically gets enough outside air for the people sleeping in it, under the conditions you measured.
• How far that typical night is from the 800 ppm goal, and where it falls on the Fresh, Building up and Stuffy scale.
• Whether your nights were consistent.
• A baseline for comparing any change you try.

It can't tell you:

• Anything about your own health or sleep. Sleep studies link bedrooms averaging 1,000 ppm or more to slightly worse sleep on average, but your nights describe the room, not you.
• Whether the air has other problems. ASHRAE notes that indoor CO₂ is not an overall indicator of indoor air quality.
• What the rest of the house is like.
• What the room will be like in a different season.

Seasons matter

HSE notes that monitoring may need repeating as the weather changes, because natural ventilation shifts with wind and with people opening or closing windows and doors. If you sleep with a window open in summer and shut tight in winter, a summer check won't tell you much about January.

Measure in the season you care about most. If your habits change with the weather, a second check when the season turns is worth the few nights it takes.