The title gives the poem its setting: someone lying awake in the dark, turning a problem over before sleep. The first lines suggest a long, drifting conversation, perhaps with another person or perhaps with oneself, about "order." That word hangs there without explanation, and the poem never tells us what kind of order is meant.
Then the images shift in a dreamlike way. A rose "collecting an alphabet" is strange but oddly satisfying. Petals layer up like letters, and a flower seems to gather language the way a mind gathers thoughts before sleep. Nature and writing blur together, as they often do when we're drowsy.
The last two lines, "night perfected / at night," loop back on themselves. Night seems to reach its fullest form only when we're inside it. The mood is quiet, hushed and a little mysterious. The speaker seems calm rather than anxious, content to let meaning arrive sideways, the way ideas sometimes do once we stop forcing them.