Not sure if this is a bug. A necromancer can cast sleep on the target and when the sleep effect wears off, the necromancer can cast sleep on the target again.

Noticed that sleep protection only kick in against physical skills. Would it be possible to have sleep protection apply against magical spell?

Being put to sleep for over 10hours throughout the duration of 3 sleep spell, while not being able to do anything and maladicted to almost dead, I don't think there is any fun factor in that.