Caution: Lemon/citrus juice is known to react in the sun (and with hormones), which can cause acid burns (proven by many dermatological studies). Personally this happened to me and several girlfriends, while using lemon juice to lighten our hair while laying in the sun at the beach. At the time, the reaction (darkly stained skin on the sides of the face and around hairline) became much more prominent in the girls who were taking birth control pills. These stains ruined a few modeling careers (tragic scarring on the girl who layed her face on a lemon soaked towel). In winter, when our tans faded, many of us noticed we had "matching stains" where the lemon juice had driped down our face.
Considering the warnings I've read from derm/cosmetologists, it's best to reserve the lemon juice treatments for nightime use only.