Sleep is an effect that renders the target helpless. A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target get a +4 bonus (equivalent to attacking a prone target). Ranged attacks gets no special bonus against helpless targets. Rogues can sneak attack helpless targets.
As a full-round action, an enemy can use a melee weapon to deliver a coup de grace to a helpless foe. An enemy can also use a bow or crossbow, provided he is adjacent to the target. The attacker automatically hits and scores a critical hit. (A rogue also gets her sneak attack damage bonus against a helpless foe when delivering a coup de grace.) If the defender survives, he must make a Fortitude save (DC 10 + damage dealt) or die.
Slapping or wounding awakens an affected creature, but normal noise does not. Awakening a creature is a standard action (an application of the aid another action).
In NWN2, even damage of 0 points will awaken a sleeping creature. Example: Cast Silence on a sleeping creature (100% sonic immunity), followed by Curse Song. Sleeping creature will take 0 points sonic damage, yet it will wake up regardless.