Um.... they did discuss it in the anime, and in the light novel.

I found this on google for someone else telling it to another person, with a spoiler tag from the light novel.

[hider=Light Novel Sleep PK]
But unfortunately, there were still some loopholes around this rule. One of them is when the player's sleeping. There are times when players, with their energy exhausted through long durations of battle, are basically unconscious when they go into a deep sleep, and unable to wake up even with some stimulations. With that opportunity, one could raise a request to duel in «Complete Decisive Mode» and moved the sleeping opponent's finger to press on the OK button. All that was left was to literally make the opponent sleep like the dead.
[/hider]