My day is your night.

Yep.

Dark.

In the night, everything is dark. (Duh, really!?)

The night is the time of the owls.
It's the time of the cats.

I like staying up late, when everything's dark already. Guess that makes me an owl or a cat or something. The night is my creative time, when I start, not finish the day.
You could also say: My day is your night.

As I start another day, I wish all of you others a good night.
Sleep well :)