Head’s Up / Heads Up

At the risk of embarrassing myself, is “head’s up” a real phrase? I’m sure that people say “just to give you the head’s up about…”, but there’s little sign of it on the Internet and now I’m worried I dreamt it one day and remembered it as fact. I was about to write “just to give you the head’s up that Donnie Darko is on BBC2 this evening” and was trying to figure out the apostrophe usage, which makes no sense in this case. The head owns the up? Can’t be. Are there multiple heads going upwards, then? I don’t understand. I’m locking myself inside a box for the rest of the day.

Donnie Darko’s on this evening. I recommend you watch it; is good.