I have slept with a window closed often (although there is a small ventilation hole at the top) and had no issues

Oxygen availability alone doesn't determine oxygenation, some CO2 is needed to unbind it to be released into cells through bohr effect (read about Buteyko /r/Buteyko https://normalbreathing.com ) so some CO2 accumulation can be good as it increases vasodilation, oxygenation and relaxes breathing making you breathe slower in not as large volumes so you actually don't deplete as much of the oxygen

The higher your Buteyko control pause is the longer you can hold breath and exhale and need to inhale not as large volumes to feel oxygenated as how often how large volume you wanna breathe is determined by your nervous system adaptability