Why is my home hotter and stuffier at night. ?

I live in a 3 fam house but am the only occupant currently in the home. I am on the 2nd floor of 3. I am self employed and home all day. Why is my home so hot and stuffy at night that I have to turn on the AC and only have fan on or windows open during the day. I don't cook and only the computer and tv were on during the day, all the lights were off.