A chart of the crossovers linking many tv series. (cited by Nev Dull)
Tommy Westphall was an austistic child on ST ELSEWHERE who, it was revealed in the closing moments of the final episode of that series, had dreamt the entire run of the show. So if ST ELSEWHERE is part of his mind, so are the 167 other series to which it is connected.
And so is Henry Fonda, who played himself in an episode of Maude. A less drastic hypothesis is that only those episodes involving St Elsewhere characters are dreams. And even that is not necessary, if the revelation did not specify that the characters (as well as the events) were unreal.