I live in an upstairs flat (and work indoors too), and although there is a shared patch of lawn at the back it's not really usable for anything more than hanging out washing, so I like the windows open to let a bit of the outside in. Sometimes I prop open the door, which is at the bottom of the stairs, too, that can get a nice breeze circulating. If I lived in a house with a private garden I'd have the back door open all summer.
I prefer the bedroom window open at night when it's hot, and so does my cat, in fact she'd like it open all year as she loves to be on the roof - we pulled it to a bit the other night, not realising she was out there, so I had to get up later as she was crying outside to come in. She can't get down from the roof unless she comes in through the (attic) bedroom window!
I recently found out that my partner doesn't actually like sleeping with the window open because he's afraid of bugs coming in!! The whole flat is like an oven in warm weather though, and the attic bedroom especially so.
