The speaker describes a life lived on a strange schedule, where the most productive hours run from 4 pm to 4 am. London at night doesn't feel familiar to them. They still haven't adjusted to it, and the city's darkness isn't the deep black they remember. Instead the sky is a "grotesque" yellow, which they compare to the colour of a hangover.
That comparison is the heart of the poem. The sky looks sickly and tired, like a body the morning after too much drinking. Yet the last stanza turns this into something almost practical. Perhaps these ugly skies actually push the speaker to work, in the same way you might push through a hangover. There's also a quiet joke in the final line: if you've spent the night under a sky this unpleasant, you won't expect much from the sunrise.
The mood is weary, dry and a little wry. The speaker sounds worn down but also resigned, even faintly amused, as if exhaustion has become simply part of the job.