"The moose sleeps at night."
Translation:Elgen sover om natten.
They gave slightly different meanings. 'Om natten' refers to nights in general. The moose sleeps at nights in this way could also be replaced with 'pleier å sove på nattestid' and have the same meaning. But if you only say 'nattestid', or 'midt på natten' you are refering to a specific time of night or even of a night