cheer up vs lighten up

cheer up and lighten up both are verbs.

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

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

Other hyponyms of cheer up include exuberate, exult, jubilate, rejoice, triumph, buoy up, lighten.

cheer up (verb)	lighten up (verb)
become cheerful	make more cheerful through the use of color
cause (somebody) to feel happier or more cheerful	become lighter
	become more cheerful
	make more cheerful