Monsters (and you!) can be put to sleep by a number of different means:
- Needles of sleeping;
- The Ensorcelled Hibernation spell, which can be cast by the player and a couple of monster types: player ghosts and some pandemonium lords;
- The Sleep spell from satyrs and Aizul; and
- The Dream Dust ability of dream sheep (unbreathing players are also immune).
Monsters are usually asleep at the time that they are generated (or "dormant", if incapable of sleep).
Sleeping monsters can be woken by nearby noise or by the player failing a stealth check, and will always wake if they are attacked (unless they are killed by the attack). Stabbing is very effective against sleeping monsters.
The player character will always wake if he/she takes any amount of damage.
- Prior to 0.15, monsters would tend to fall asleep if enough time has passed since they last encountered the player.