During the recording of Sgt. Pepper's Lonely Hearts Club Band, John had become quite lazy. Paul was cracking the whip to get the others into the studio, while John often sat at home and watched TV in bed. Song ideas came from anything which was immediately availible to him - he rarely ventured out of the house. Such is the case with Good Morning Good Morning, the chorus of which came from a television commercial for Kellogg's Corn Flakes.
The Corn Flakes jingle
The main jingle of the Kellogg's Corn Flakes commercial that inspired "Good Morning Good Morning": "Good morning good morning, best to you each morning. Sunshine breakfast, Kellogg's Corn Flakes, crisp and full of fun."