For a long time, I was unaware of a common misconception: ductless air conditioners do not provide ventilation for rooms. Some background behind this experiment:

• After I moved to a newer apartment, turning on the air conditioning to sleep always resulted me in waking up constantly feeling groggy.
• Newer apartments have very tight window seals and doors obviously shut more tightly.
• My old apartment even had vents at the bottom half of the room doors that looked something like that:

• Goes without saying cold air is leaking even if I shut my windows and doors while using the air conditioning in the older apartment (thus it has better ventilation?).

Without further ado, below is the data that I have recorded with my Raspberry Pi under different circumstances.

Fully closed windows and door

Leaving a small crack on a small overhead window

Promotes convention, since the indoor unit of ductless air conditioners take takes in the warm air above it to be cooled.

Looking at the data, a small crack in my room's overhead window causes CO2 levels to cap around 1000ppm, which is not the best but is still at more acceptable levels than before.

Now, I mostly sleep with the windows opened and with only a fan turned on.

Check out the code for my Raspberry Pi air meter here.