or everything in moderation.

or Moderation is the best policy
are the translations of "l'excès en tout est un défaut" given on the site:
http://www.e-frenchtranslation.com/fr/proverbes.htm
ça vaut ce que ça vaut !

Often appears with "everything in moderation". I believe it's a Greek proverb, which makes me think of the golden mean.

Googling, I found the opposite of what you need, Blake's "the road to excess leads to the palace of wisdom".

my two cents!

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Note added at 1 hr (2006-03-08 19:29:57 GMT)
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oops, everytHing, of course.

These are idioms which express the idea that less is more or that anything in excess is bad for you.

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Note added at 1 day6 hrs (2006-03-10 00:20:45 GMT)
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I have been thinking about this image of "trop nuit" If this is a poem and the image of too much night is being used to suggest the idea of "too much of a good thing is not good for you perhaps you could retain this imagery and say something like " too long is the night". If is a poem you want a sucint way of expressing it and it depends on how strong the image in the poem is.