Anecdotal, but I'm convinced it screws up sleep quality. I'd just accepted for the longest time that waking up groggy with a slight headache and tired was the norm until I put a CO2 monitor in my room. With the door closed, it climbed up to 1500ppm in under an hour.

I'm certain many people are sleeping in similar conditions without realising and ventilating their rooms properly or leaving the door open.

Higher CO2 vs. free cat access at 4:17am. No win scenario!

And window.