lighten has 4 distinct synonyms spread across 5 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are weigh down, darken.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
make more cheerful
“the conversation lightened me up a bit”
reduce the weight on; make lighter
“she lightened the load on the tired donkey”
become more cheerful
“after a glass of wine, he lightened up a bit”
make more cheerful through the use of color
“The paint will brighten the room”
Synonyms, antonyms and the broader and narrower relations for lighten come from Princeton University’s WordNet. Full sources and licences.