Insomnia is an ability introduced in Generation III. The Pokémon cannot be put to sleep; this includes both opponent-induced sleep, as well as user-induced sleep via Rest. If the Pokémon tries to use moves such as Rest, that move will fail. It is one of two Abilities that prevent sleeping (the other is Vital Spirit).

Pokemon

Pokédex	Pokémon	Sprite	Type	Obtained
#096	Drowzee			Natural
#097	Hypno			Natural
#163	Hoothoot			Natural
#164	Noctowl			Natural
#167	Spinarak			Natural
#168	Ariados			Natural
#198	Murkrow			Natural
#225	Delibird			Dream World
#353	Shuppet			Natural
#354	Banette			Natural
#430	Honchkrow			Natural