おやすみ

How it works

Two thoughts sit side by side: the set greeting おやすみ (good night), then the reason for it. もう (already) pushes the adjective 遅い(おそい) (late) into the “it's gotten late” sense. There's no です (is) after the adjective, so the whole line stays plain and casual. The final よ (I'm telling you) adds gentle emphasis — you're pointing out something the listener should take note of, like “it's late, you know.”