Three short lines give us a small, quiet scene: someone awake after the rest of the world has gone to bed, in a room where a song is playing. The title, "Up late," sets the hour, and the poem never says more than it needs to.
The most interesting phrase is "night work / against tomorrow." It suggests the speaker is doing something now to prepare for, or push back against, the day ahead. That could be an actual task, or it could be the quieter work of staying awake, thinking, or making something. The poem doesn't say, and that openness is part of its charm.
The mood is calm, private, and a little wistful. The song gives the room warmth, while "against tomorrow" adds a faint pressure, as if the night is something to protect. The speaker seems content but aware that morning is coming. Judging by the wording, the feeling is less worry than a gentle, stubborn hold on the last hours of the night.