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.

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.

im english, there's "sleep on it"