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Wild cat and mouse Pokémon including Pikachu are mostly nocturnal. Conditions for finding food are more favourable at night. Cats and mice that run around fields and mountains in broad daylight are mostly restricted to fiction like anime, films and TV.
Typically, cats and mice sleep in the day and move about at night. By the way, humans are creatures that do things during daylight. As such, Pokémon that are kept by humans naturally adjust to being awake during the day.
You'll notice this with Pokémon that live in the zoo, but Pokémon who have adjusted to daylight hours sleep shrewdly at night. This is because within the habitat of their cage or a Monster Ball, they can rest without having to fear being attacked by predators. However, for those Pokémon who aren't in a Monster Ball, the night is naturally a dangerous time.
After all, the night is their active period.
