Sleep soundly, my plain sister! At night, I am a roaring river.

This couplet is a playful yet firm instruction. Imagine a parent or an older sibling telling a young girl, 'Go to sleep now, you silly sister!' It hints that night brings a different atmosphere, perhaps a bit serious or vast. The line 'At night, I am a fearsome river' suggests that the world, or even the speaker, transforms with the darkness. It's a gentle nudge for the child to settle down, hinting at the big, mysterious nature of night, or simply playfully stating that playtime is over and it's time for rest.