If I understand correctly, you want to prevent the device from going to sleep so you can play your video without interruptions. You can have your app prevent the device from "going to sleep" like this:
[UIApplication sharedApplication].idleTimerDisabled = YES;
Just remember to set it back to NO
when you are done playing your video like this:
[UIApplication sharedApplication].idleTimerDisabled = NO;
I don't know how you are playing your video, but try you can probably just call those functions before you play your video, or immediately after you are done.