1
general::
Phrase(s): lighten up (on someone or something)
to be less rough and demanding or rude with someone or something. (See also lighten up.) • Please lighten up on her. You are being very cruel. • You are too harsh. Lighten up., Phrase(s): lighten something up
to make something lighter or brighter. • Some white paint will lighten this room up a lot. • The sunlight came in and lightened up the kitchen., Phrase(s): lighten up
to become lighter or brighter. (See also lighten up (on someone or something).) • We applied a new coat of white paint to the walls, and the room lightened up considerably. • The sky is beginning to lighten up a little.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Become or cause to become less serious or gloomy, and more cheerful. For example, Lighten up, Sam? it'll turn out all right. This slangy expression transfers reducing a physical weight to a change of mood or attitude.
American Heritage Idioms