Applying lemon on the face overnight is not recommended. Lemon is an acidic fruit and can cause skin irritation, dryness, and sensitivity to sunlight. If you want to use lemon for its potential skin benefits, it's best to dilute it with water or a carrier oil, and apply it for a short period of time, such as 20-30 minutes. It's also important to protect your skin from sun exposure after using lemon, as it makes skin more sensitive to UV rays. It's always a good idea to patch test a small area of skin before using lemon on the face or anywhere else.