The speaker is calling a lover to come close in the middle of the night, and the whole poem is shaped by a sense of urgency. Night is the one time the two of them can be together without interruption, and the speaker knows it won't last. The moon becomes a symbol of their love, steady and present whatever the hour, month or year, while the sun stands for everything that threatens to pull them apart: noise, busy people, and perhaps second thoughts ("I know we'll change our minds").
The speaker's feelings mix tenderness with anxiety. They love the quiet, the sound of the lover's breath, and the "midnight skin" they find so beautiful. Yet they admit that missing a kiss before sunrise might be their "undoing." The repeated "come closer" works like a plea that grows more desperate. The mood is intimate, romantic and slightly sad, like a secret held against the clock.