Learn French: Bonne nuit, chéri. - Good night, darling.

In French, bonne nuit is strictly used as a farewell right before someone is going to sleep. If you are parting ways in the evening but they are not going to bed yet, or if you are greeting someone in the evening, you must use bonsoir (good evening) instead. English speakers often make the mistake of using bonne nuit too early in the night!

Why is it bonne and not bon?

The word for night, nuit, is a feminine noun. The adjective must agree with it, changing from the masculine bon to the feminine bonne. This also changes the pronunciation from a nasal vowel sound in bon to a strongly pronounced "n" consonant in bonne.

How does the spelling change if I am saying this to a woman?

You would add an "e" to the end of the term of endearment, making it chérie. However, the pronunciation of the masculine chéri and the feminine chérie is exactly the same.

Could I also say mon chéri in this context?

Yes, it is very common and natural to include the possessive word and say Bonne nuit, mon chéri (or ma chérie for a woman). Both with and without the possessive are widely used by French speakers to mean "darling" or "my darling".