Watermelon can be eaten overnight if properly preserved, but it is not recommended.
Watermelon is a common summer fruit, which contains a lot of water, sugar, fiber, potassium, vitamins and other nutrients. If overnight watermelon is properly preserved (e.g. refrigerator chilled, plastic wrap sealed, no nibbling, etc.), and the pulp is not deteriorated (e.g. no odor, no change in flavor, no bacterial spots, etc.), it can be eaten.
However, watermelon contains a large amount of water and sugar, cut overnight, it is very easy to lead to a large number of microbial growth and reproduction, consumption is prone to gastrointestinal inflammation.
Secondly, the vitamins and other substances in watermelon can be partially oxidized by the oxygen in the air after overnight, thus losing their original effect, so it is not recommended to eat overnight watermelon.
If you eat overnight watermelon and experience discomfort, go to your local hospital immediately.