I don't think this would be popular even for me but a mechanic that would more or less debilitate players stat wise and graphically if they didn't sleep at all.
Not sleeping for an extended period of time would create black corners on your screen and slowly, ever so slowly, darken until you collapsed from being so utterly tired. The effect would also create a kind of tunnel vision.
Stat wise, you would be a little bit slower, your bite attacks only do 75% of the total damage
The duration of time needed for this to take effect would have to be at least 1.5 days until your dragon starts exhibiting symptoms of bone dry tired-ness.
The duration it would take to collapse completely would probably be around 15 minutes are so if you didnt find a place to sleep.