Just read this article by a researcher at MIT. Students and parents especially should read this as it affects cognition and alertness levels. Students in Singapore typically have preciously little time (usually less than 8 hours) to sleep, hence the quality of sleep is very important.
Basically to summarize, sleeping in a closed bedroom (without ventilation, with windows and door closed) leads to high carbon dioxide levels that can possibly affect the brain in negative ways (decreased cognitive function). Switching on air conditioner should be fine as essentially fresh air is introduced from the outside.
The solution can be as easy as opening the door/window. Open window is said to be better than open door. However in Singapore, light pollution is another potential problem, most likely there is some form of light shining through the window at night. Singapore’s doors and windows are not as airtight as those in winter countries, but it seems there is still some cause for concern.
A scientific paper for those who want to read in greater detail: https://onlinelibrary.wiley.com/doi/pdf/10.1111/ina.12254.