lighten up vs light up

lighten up and light up both are verbs.

Word	Noun	Adjective	Verb	Adverb
lighten up	No	No	Yes	No
light up	No	No	Yes	No

As verbs, light up is a hyponym of lighten up; that is, light up is a word with a more specific, narrower meaning than lighten up:

Other hyponyms of lighten up include irradiate, illume, illuminate, illumine, light.

lighten up (verb)	light up (verb)
make more cheerful through the use of color	introduce light into
become lighter	begin to smoke
become more cheerful	become clear
make more cheerful	start to burn with a bright flame
	ignite