I was going to include this in a longer bug report, but it's occurred so frequently and caused numerous gameplay issues that I decided to report it separately. Ascended Sleepers cause several graphical problems when killed, due to their death animation. Most noticeably, all dynamic lighting gets deactivated. Torches and Light spells stop working, and spell effects no longer illuminate the area. The interior map (F4, leftmost tab) also stops working, and just shows a blank page. The only workaround I've found is to save the game, quit to the main menu, then reload.
The death animation itself also has some strange things going on with it. The sleeper's body portion will stretch outward to infinity and then fade from sight, but it seems the game engine is still treating it like it's there, since this will cause slowdowns in performance if it happens outdoors. The slowdown is so bad that every normal attack I attempt turns into a power attack due to the processing delay. About half the time, the sleeper's skull will sink through the ground and become inaccessible, similar to the glitch in daedric ruins where enemies fall through the floor when killed.
I tried searching for possible fixes, or if this has come up before. I saw that this was posted in 2011, but the file no longer exists.