Sleep causes the victims to immediately fall into a deep slumber.
When a wizard casts a Sleep spell, he causes a comatose slumber to come upon one or more enemies (other than undead and certain other creatures specifically excluded from the spell's effects). All creatures to be affected by the Sleep spell must be within a 30-ft. radius. Creatures in the area of effect must make a Saving Throw vs. Death with a -3 penalty or fall asleep. Monsters with 4 Hit Dice or more are unaffected. Attacks against sleeping opponents never miss.
Contrary to the description and common sense, the effect of the spell are more equal to paralysis or a coma rather than sleep, as affected creatures will not wake up when attacked. This is likely due to the limitations of the game engine. As a result, Sleep is essentially equivalent to the Hold Person spell, but can affect infinite amount of creatures, works on creatures that don't count as a "Person", has a harder saving throw and is available on level 1. The only downside is that the enemy needs to be level 3 or below, which mostly limits its use to Baldur's Gate. Hovewer many strong enemies(such as Ogre Berserkers or Sirines) are still affected.