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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).