"At night you all sleep."
Curiously, this is pretty natural for Spanish speakers (and I suppose that for other latin-derived languages too). "En la noche dormimos" is more common and natural than "En la noche todos nosotros dormimos". I love how many things from Latin are still alive and in use today in it's heirs.
I think "omnis" is singular, with the sense of the whole of something. Here we need "all" as plural. There is the sense of "all of you", that is, more than one. The plural is, I think, "omnes".
I see alterprete on this page gives "Noctu vos omnes dormitis" for "At night you all sleep" so I seem to be right.
THE WORD "NIGHT": in another exercise Duo shows a picture of the moon at night with the caption "night", not "at night". The expected answer is "noctu". "Noctu" is an adverb meaning "at night". The noun for "night" is "nox". I've tried to draw Duo's attention to this as I think it could cause confusion.