The right indoor climate for a good sleep
A good temperature range to fall asleep in is 18 to 20°C: outside of this range it can be difficult and of course every individual is different.
Having a sleeping environment that is draught free and has the right humidity level will also make a big difference to a persons sleep quality.
A draught while you’re sleeping can actually contribute to irritated eyes, and muscles stiffness.
Also the room needs to be dark and noise levels need kept to a minimum.
An open window, a simple solution
A sleep experiment showed that the CO2 levels during the night ranged from 1,700 to 3,900 ppm when the window was closed, and when the window was open, the CO2 levels were 500 to 800 ppm (with reference outdoor CO2 levels of 660 ppm). With windows open, sleep latency also decreased (i.e. how long it took people to fall asleep); however, noise and draft from open windows may counteract the positive effect of better air quality.
From The effects of bedroom air quality on sleep and next-day performance by P. Strøm-Tejsen, D. Zukowska,P. Wargocki, D. P. Wyon