Vital Spirit

This Pokémon cannot be asleep. This causes Rest to fail altogether. If a Pokémon is asleep and acquires this ability, it will immediately wake up; this includes when regaining a lost ability upon leaving battle. This ability functions identically to Insomnia in battle. Overworld: If the lead Pokémon has this ability, higher-levelled Pokémon have their encounter rate increased.

The Pokémon is full of vitality, and that prevents it from falling asleep.