I tested it deep into strongholds (it was a while back but I think it was 55-56 Amaal'ja), many casts were completely naked and there was no difference at all in duration when it landed, not even a few seconds. That means that either A) The required Enfeebling Skill for capping sleep on most Sleepable enemies is so low that you don't need Enfeebling gear anyway or B) Enfeebling doesn't do anything at all for Sleep.