Sleep is a Status Effect. An enemy under the affects of a sleep power will fall asleep. A player or critter who is asleep is immobilized and unable to use any powers. Break Free inspirations and Domination can be used to escape. Sleep effects end early if the victim suffers damage, gets healed, or gets affected by a Knockdown/up/back or Repel effect.
Magnitude, Resistance, Protection
Except for Slows, all status effects work the same way. The target has both Resistance and Protection. In order for the attack to take effect, it must have a big enough Magnitude to overcome the Protection. Once the Protection is overcome, the duration of the effect depends on the target's Resistance. Attacks can stack in order to overcome a target's Protection. No Slow Protection exists, only Resistance.
PVE: Minions have a Protection of 1. Lts are 2. Bosses are 3. Elite Bosses are 6. The Purple Triangles of Doom on an Archvillain or Hero add 50 to all but Sleep while they are up. Some specific enemies are resistant to fear.
PVP: As of Issue 13, Protection does not exist in PVP, only Resistance.
How to tell if a power uses sleep
|Havoc Punch Melee Foe Sleep Mag 2|
This power is found in the Electrical Melee power set available to Brutes, as well as in the Blaster Electricity Manipulation secondary.