"light up" & "lighten up" 之間的差異 (by Andy Lee)
(1) To light up is to make something brighter or happier.
(A) "They light up their house with beautiful lanterns whenever they have parties." (Literal)
(B) "She lights up the room with her personality." (Figurative)
(2) Lighten up is to become less sad or less serious, usually without an object, "You are always so serious. Can't you just lighten up?"
用法（b）及（2）可能是非常相似和混亂，但我認為 “light up”（二）是更積極主動，而“lighten up”是相對被動。我從來沒有太注意這兩個字的差別，而且我相信大多數人也沒有注意到其差異。儘管如此，基於我的研究，嚴格來說，這兩個動詞之間是有著細微的差別的。
The usage (B) & (2) could be very similar & confusing, but I think "light up" (B) is more agressive, while "lighten up" is relatively passive. I never pay much attention to difference of those two verbs, and I believe most people don't notice it that much either. Still, based on my research, strictly speaking, there is a slight difference between thiese 2 verbs.