How do I put the "sleep-mode" off?

Gah, I’m getting mad here. I just putted on a file to download over the night, and when I woke up the computer was in “sleep mode”. How do I turn this off? I mean, if I let the computer be on, I want it to be on, not go to some other mode.
Btw, I’m using Windows Vista. I’d be very happy with an answer. Thanks! ;D

go to control panel, power options, under the plan your using click “Change plan settings”.

Thanks alot dude! <33

No Problem, Glad to help.