The poem starts with something small and physical: a sleepless night, bitten by fleas, with a cold that seems to run through the speaker's whole body. It's an irritating, helpless moment, the kind where you can't get comfortable and can't think about anything else.
Then the poem turns. The second stanza shifts to "the poor" and "the power," and the fleas begin to look like a picture of those who hold authority. They itch, tick and eat whenever they like, and they're described as "good-natured" about it, which is pointedly ironic. Meanwhile the poor appear only on Sundays, as if their freedom or visibility is rationed.
The speaker's feeling seems to be a mix of discomfort and quiet anger. The tone is dry and understated rather than shouting, which makes the comparison sting more. The mood is bleak and itchy: a small bodily misery that gets stretched into a comment on who is allowed to feed on whom.