Sleep effects cause a standing sleep-like state, where the character is unable to act for the duration of the ability or until dealt damage of any kind (in contrast to Stun where damage doesn't break the effect). Sleep effects are removed by the Undead ability, 
[Will of the Forsaken].
Examples of abilities that cause sleep effect:
Various items can also cause the Sleep effect, including 
[Slumber Sand].
Any sleep effect, no matter the duration listed, will last a maximum of 15 seconds when used against players in PvP.
See also: Crowd Control, Charm, 
[Dazed], Fear, Root, 
[Slow], Snare, and Stun.