Koltira is undead; as such, he neither needs to sleep, nor is he in the habit of doing so (he's capable, but sleep was forbidden while he was enslaved/later imprisoned).

What he DOES need is to fight, so he'll be eagerly seeking nightmares of any kind to slay. He's an excellent tank, and would be glad (for a very gloomy value of the term) to help/protect others in this endeavor as well.