The speaker describes lying awake at night and listening, not to the outside world but to the sounds inside their own head and body. Night isn't peaceful here. It's "a dull noise in my ears," a low hum that never quite switches off.
The comparisons are what make the poem interesting. The noise is like "the bark of a conference," a funny and slightly uneasy image of many voices talking at once with no one really listening. Then it becomes "the body's own talk," as if blood, breath and nerves were holding a conversation of their own. The last lines say this inner chatter is "exact as a question / in mathematics," which suggests something precise and unrelenting, a problem that demands an answer but may not have one.
The title, "Default mode network," points to the brain's resting activity, and the poem seems to take that idea in. The mood is quiet, alert and a little restless: calm on the surface, but with a mind that won't stop working.