I can confirm that turning a monitor on its side does result in distorted colors, especially on the edges. When I experienced this with a Commodore 1701 monitor (we're talking early to mid-1980s) the only other electronic device nearby was the Commodore 64 it was attached to, so it's unlikely caused by other devices.
I believe that came later. I do recall my early IBM PC compatible having a monitor with a degauss button that would give a satisfying FOOMP sound (accompanied by a twitch of whatever was on the screen) if the monitor had been sufficiently gaussy before.
I'm working from old memories here but iirc the degaussing circuits in TVs and monitors only work after they've cooled down. He didn't have to leave it all night, maybe an hour if that would have done.
This thread explains that you need to degauss the monitor after turning it on its side: http://forum.arcadecontrols.com/index.php?topic=84041.0
I suspect leaving the monitor off overnight effectively degausses it.