Only the Sleepless Know the Length of Night in Chinese / Japanese...

不眠之夜长久交知人心 literally translates as: [Only one who does] not sleep, learns how long the night is; [Only by] long acquaintance [does one] learn a person ['s true] character.

Basically, this proverb suggests that we really need to experience something intimately and for a long time to really know everything about it.

This can also be translated as “Spending years with someone is the only way to know them.”

Note: Sometimes this proverb is split into just the first or second idea alone (first 5 or last 5 characters only).