Here’s another genuine curveball. A good friend told me to never clip one’s nails at night. He openly said this is a Korean myth, but said you should do it anyway.
The logic is that at night, the spirits of your ancestors are active and take care of you. Hence damaging yourself offends them, or conversely grooming yourself is unnecessary.
The obvious problem then is what else should you not do at night? Can I take a shower? Brush my hair? Wash my hands?