To light up is to make something brighter or happier. "They light up their house with beautiful lanterns whenever they have parties." (Literal) "She lights up the room with her personality." (Figurative) Cheer up vs. Lighten up Cheer up is to make something happier/ not sad or to become happier / not sad. Lighten up is to become less sad or less serious. Cheer up can be with or without an object. "They cheered Sam up by taking him out to a fantastic night club." "Sam cheered up after spending time with his friends." Lighten up is usually without an object. "You are always so serious. Can't you just lighten up?" But I think the non-human object works here: "We have to lighten up the conversation because this topic is too depressing for me." But not here: **We want to lighten him up since he's so sad.*** (This is wrong to my US English ears. I would use 'cheer up' instead.)

I assume you mean "lighten up" and "cheer up". Lighten up - be less serious Cheer up - be less sad "Light up" means to suddenly become brighter. "Her eyes lit up at the mention of his name."