Sleep

Sleep is a rare status effect that completely disables those affected 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 its target invulnerable for one second and Song of the Siren will do so for the entire duration. Cold Embrace will render its target invulnerable only to physical damage.