ok, the sky box may be overheating, I'm assuming here, as you havn't said if the TV or the box is what is specifically switching off. I've had this happen on a DVR before, with an X-box sitting on top of the vents, make sure the boxes have access to ventilation, TV not over a fireplace or mounted too close to the wall, i.e. no gap...
Sleep on the TV could be a button or a menu selection, you'll have to read the manual for that. can't help you any more than that.
If your sky box is what is crapping out, it may be a bad heat sink or a fan has failed. causing a thermal overload and so a shutdown. then it's time for repair or replacement.