"Late night" is a single sentence, but it holds a whole attitude toward the hours most of us sleep through. The speaker is awake, and instead of treating that as a burden, they treat it as a privilege. The darkness isn't empty or frightening here. It is "filled with wonderful things," a place crowded with something worth seeing.
The poem never says what those things are, and that choice matters. We can imagine stars, quiet thoughts, strange shadows, or a feeling of having the world to yourself. Leaving it open lets each reader fill the dark with their own wonders.
The speaker's tone is quietly pleased, maybe a little secretive. There's a gentle sense of pity in "people who sleep," as though they are missing out, though not in a harsh way. The mood is calm, hushed and curious. It turns being awake at night into a small kind of discovery.