Sleep is an exclusive metablast; an attack that causes no normal damage. Instead it causes the loss of three shots if it would ordinarily have inflicted any damage. If the damage matches the target's Willpower, they fall asleep. A sleeping target will be awakened by any damage or by vigorous shaking, but not by loud noise and such.
This is basically a specific variant of the Stun metablast. Damage Immunity to Influence or Life protects against sleep.
An exclusive metablast has none of the normal effects of that blast, including any other metablasts that would ordinarily apply. In effect, an exclusive metablast is a separate effect that replaces what the blast normally does. Still, it takes it's appearance from the blast it is used with, if it does any damage it is of the same type, and a Damage Imunity that would protect against the blast also protects against the exclusive metablast.
If you must always use this metablast, and thus cannot do normal damage, the metablast schtick is free.
Sleep spells can be wondrously effective ways for character's to be captured, if the script requires them to be. Don't make a fuss if your GM causes you to fall asleep under the power of the script.