You haven't mentioned what model / brand the TV is, however the symptom you mention would suggest it as being a CRT type (not flat panel) and the fault could be caused through magnetisation "if" you have audio speakers used for a hi-fi etc sitting anywhere near either side of it, the TV has a demagnetizer circuit built in which could be slowly bringing the picture back to normal after switch on only for the problem to re-occur again after its switched off during the night. (if applicable move the speakers away!!) Another odd reason can sometimes cause this to happen if the set has been sat facing down on the tube face whereby the correction magnets around the tube slide slightly, these requiring resetting and locked. If by any chance it not a CRT set but a flat panel then its anyone's guess as to the reason, especially as you haven't said what it is.