Anyone else tends to be bothered by the idle sleeping animation because it means having to type /sit again to keep a cool (com?)posure in Haven? Why not give it an alternate sleeping version for sitting down?
Something as simple as making the head bob and use the Zzz effect would do the job nicely enough, so long as the animation isn't overlapped back into standing up again once you move the cursor again to wake up the knight. Actually sitting and lying down on the guild hall's furnitures happen to not trigger the AFK animation, come to think of it.
Mind this: The sleep animation is used to show that players are AFK. Why do you need this? Are you simply staring at the monitor for five minutes doing nothing? -1 because there's no point. If you're in another browser and you're sleeping when you come back, it's not that hard to type /sit.