A wand of sleep puts monsters it hits to sleep.
You can use it to help defeat (or escape) difficult monsters (unless the monster has sleep resistance); just be careful where the beam bounces. In a coaligned temple, you can use this wand to park a heavy monster on the altar so that you can kill it there without having it run away from you.
Bolts of sleep bounce from walls and unidentified secret doors, but are absorbed by non-secret or discovered secret doors.
When engraving, this wand gives the same message as a wand of death. If the wand was not generated in a monster's inventory, the odds are 6:1 it is sleep.
Wands comprise 4% of all randomly-generated items in the main dungeon, 6% in containers, 5% on the Rogue level, and 8% in Gehennom. There is a 5% chance that a randomly-generated wand will be a wand of sleep.