I taped a window shut to install a portable A/C, but it feels wrong to run it at night when the outside air is colder than inside. It can cool relatively effic

I taped a window shut to install a portable A/C, but it feels wrong to run it at night when the outside air is colder than inside. It can cool relatively efficiently in that case, so it doesn't need to run for long, but ventilation would make it free.