lighten up in WordNet English dictionary

1. make lighter or brighter
   "The paint will brighten the room"
   brighten  lighten up  lighten 
   
2. become more cheerful
   "after a glass of wine, he lightened up a bit"
   lighten  lighten up  buoy up 
   
3. make more cheerful
   "the conversation lightened me up a bit"
   lighten  lighten up  buoy up 
   
4. become lighter
   "The room lightened up"
   lighten  lighten up 
   

lighten up in English Wiktionary