The purpose of having characters rest periodically is to encourage players to get up and move around every so often instead of just sitting at the game non-stop. Therefore, I think we can safely say that real world time is the primary deciding factor and the character's physical fatigue is not the main issue. Also, if I understand correctly you can only sleep during the night time part of the cycle anyway, and resting is not required it only gives certain bonuses if you do. Maybe someone else knows more or knows better.