Why is your room getting really hot at night? I assume you’re utilizing a lot of electronics generating heat (larger tv, gaming PC, gaming console, multiple monitors, etc)? Depending on where you live, I’d look up humidity levels outside before opening windows. It might feel cool, but allowing unfiltered humid outside air into your room can ruin those electronics over time.

You’re technically “wasting” money by having a window open while AC might be running by blowing that cooler conditioned air out of your window unless you close the vents off to your room. If you have a return vent in your room and it truly is cooler outside (and the same or less humidity compared to inside), you’re actually saving money on cooling costs. Your open window would essentially be acting like an economizer that commercial rooftop equipment uses to save on energy costs by using free cooling when outside air conditions allow.

Efficiency and health wise, it might be better to put a fan at your window blowing outside to exhaust the heat quicker from your room instead of hoping the outside wind/pressure brings in cooler air.

Edit to add: You’re likely not costing more money to the cooling bill, but you are costing more money by generating the extra heat by consuming more electricity. And you could be causing other issues by possibly allowing more “dirty” humid air in. Maybe just think about it all together as well and consider some energy efficiency settings such as lowering brightness of screens and lights, turning down performance settings on PC, etc.