From Dungeons of Dredmor Wiki

Sleep is a non-damaging effect that is found as a side effect of several Spells and Traps, and can be found as an enchantment on Weapons. Affected targets will be placed into a "sleeping" state (where they do not move or attack) for a brief period of time or until they are awakened by being attacked, or by the sounds of fighting nearby. Sleeping creatures can be identified by a series of Z's above their head.