This is a very common proverb. In my own language I've heard it often as ‘Een zuiver geweten is het beste oorkussen.’ which means the same as the P./E. sentences here. But it also exists in German as ‘Ein gutes Gewissen ist ein sanftes Ruhekissen.’ (A good conscience is a soft pillow.) I've also heard an amusing English version: ‘A quiet conscience sleeps in thunder.’

Beginning to think that the German version was probably the source of the Dutch version since the German version rhymes and is more pithy, and already having spent way too much time trying to look up the others in worthless on-line proverb dictionaries that all copied each other, I looked up the German one and found this in Latein-Deutsch: Zitaten-Lexikon: Quellennachweise:

qui sibi nil conscit secura mente quiescit.

(Who has nothing on his conscience, sleeps with an untroubled mind.)

Apparently, this is from a 1521 Christian text called Loci Communes, or Common places, so it probably was already well-known at the time. Unfortunately, the author was German so we cannot know if the Latin version precedes the German proverb, or if the author coined a well-known German proverb into a Latin phrase.

I never heard the exact sentence, but "dormir com a consciência tranquila"(more common) or "dormir com a consciência limpa" (less common) are very used. The idea is quite the same.

One can't sleep well with "peso na consciência" or "consciência pesada". (lit: with weight in conscience or with a heavy conscience)

In my region i've never heard of that, but searching on the net i learned it is a latin proverb.... maybe some people use that in Brazil.

I kinda like it!

I don't know about consciência, but conscience (sense of guilt), consciousness (awareness), and conscious (awake), all have totally different meanings in English. Conscience is the correct word for the above sentence.

I agree with duolingo that only conscience makes sense here. Consciousness has a different meaning than conscience. Consciousness is the sense of being awake. If you find someone lying on the floor who is not responding to your words or touch, this person is unconscious, or not conscious. He has no consciousness.

If you look for ‘A clear conscience is the best pillow.’ you get a lot of hits, so it seems that this is a common phrase in English as well. Another variant I've seen is ‘A quiet conscience sleeps in thunder.’ (Which I found quite amusing.)