[MITSardine]

In French, there's a saying: "la nuit porte conseil". Roughly translates to "the night advises", and it means it might be better to sleep on it.

I recall my father (also a mathematician, incidentally) often repeating this to me.

[toyg]

Literally the same in Italian, la notte porta consiglio. It's in the Bible, in nocte consilium from the Book of Proverbs, but it's likely to pre-date even that by centuries.

[narag]

In Spanish "consultar con la almohada" (consult with the pillow)

[lesostep]

Bulgarian "утрото е по-мъдро от вечерта" (Morning is wiser then evening)

[8note]

im english, there's "sleep on it"