Sleep

Sleep is a rare status effect that completely disables those effected for the entire duration. However, Sleep can be broken by dealing damage to the affected unit. Nightmare has the additional effect of disabling any units that damage the Sleeped unit with a physical attack.

An interesting note is that if a unit is invulnerable sleep cannot be broken. Nightmare will render it's target invulnerable for one second and Song of the Siren will do so for the entire duration. The only way to break sleep on an invulnerable unit is by Purging them.