Birds hunt for food at night, but this also attracts bats that hunt for insects at night. Field mice are mostly small in size. To avoid being preyed on, they often come out to forage at night, which has led to owls hunting for rodents at night. The pygmy monkey is small and weak, lacking effective self-defense capabilities, so hunting for insects at night is relatively easy and safe for it.
The formation of nocturnal behavior is a manifestation of ecological niche differentiation in animals. For some animals, this is a way to avoid predators; but for others, it's a way to forage for food.