The creature is helpless.
The creature can’t take actions.
The creature takes a -5 penalty to all defenses.
The creature is unaware of its surroundings.
The creature falls prone, if possible.
The creature can’t flank.
This condition applies when a creature is knocked out, cast into a magical slumber, or otherwise incapacitated totally.
A creature that has fallen asleep naturally—as opposed to being knocked unconscious by a power or other effect—is unconscious but not totally deprived of awareness; it can use its passive Perception to hear things, but with a -5 penalty.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 223, Rules Compendium, page(s) 234, Monster Vault: Threats to the Nentir Vale, page(s) 125.