When a character is afflicted with sleep, they become completely immobile and will take an even longer time to recover from it than being frozen. However, a character is made to wake immediately upon being hit. A sleeping unit can remain asleep longer if they are hit with the condition again while already asleep. Ein is immune to Sleep. Lina and Fia have no resistance to Sleep.
In Yggdra Union - We'll Never Fight Alone, sleep behaves differently depending on circumstances. Generally, when a unit is asleep, they are unable to engage, thus rendering them mere targets for a one-sided defeat. This issue is demonstrated most often with the Skeleton (Class). On field, a sleeping unit cannot move at all until awoken, and can easily be dispatched by opposing forces. They can still join a union if the opposing forces intiates an attack and they are aligned within the formation, but not when allied forces attack (for obvious reasons).
Some Cards have the power to inflict Sleep. When the skill is executed successfully, the battle will end with a defeat on the victim.
In Gungnir, the afflicted is completely incapacitated, but they will heal small amounts of HP gradually. Sleep goes away on its own, but can end if the sleeping unit is woken up by an attack.