While I appreciate natural remedies that work, I cringe a little when I see non-experts recommending lemon juice as a skin brightener. The peels of certain citrus fruits contain furanocoumarins, which are organic compounds that can react when exposed to the sun. This reaction is called phytophotodermatitis and can cause redness, blistering, and long-lasting hyperpigmentation. Furthermore, lemon juice is acidic with a pH of 2-3 so it can disrupt the barrier and irritate the skin. It also contains limonene and citral which can be irritating and cause contact dermatitis.
If you are looking for a skin brightener, opt for a vitamin C serum specifically formulated for the face instead. Some of my favorites are Skinceuticals CE Feurelic, SkinBetter Alto Defense, and Isdin Melaclear.