When you whistle at night you attract snakes to your bed.
My informant first heard about this proverb from her mother. Due to this superstition my informant never whistled at night as a child as she did not want to bring harm to her family. She lived in a small country town in Korea, thus this environment helped this superstition gain steam. In her specific city, snakes were prevalent all over the neighborhood. She believes now that this proverb was told to discourage children to make noise at night, as this bothered the neighborhood. She also once witnessed her sister whistling while she was about to go to sleep, my informant then quickly told her the proverb so she would not brings snake into her bed.
This is actually quite an interesting usage of folklore as it was used to discourage children from doing a certain thing that bothered other townspeople. It also makes sense as snakes were prevalent all over the town and children usually hate snakes, thus the adults used a familiar animal to discourage children from whistling for generations as adults constantly passed this folklore down.