A person sits down to write letters in the morning but keeps drifting toward sleep. The poem catches that half-conscious state in just a few lines, and the speaker seems more amused and resigned than upset about it.
The most interesting choice is the image of the "cotton shop of sleep." Sleep becomes a place the speaker keeps wandering into, a store stocked with soft, white, muffling material. Cotton suggests padding, warmth and quiet, so the drowsiness feels cozy rather than painful. The phrase "all morning" tells us this isn't a single slip but a steady pull, and "several times" shows the speaker repeatedly losing the fight.
The last line brings us back to something ordinary: letters, a small act that needs attention. The nod comes "in the middle," so the sentences are probably left unfinished. The mood is gentle, slow and a little humorous, and the short, loose lines mimic the dozy feeling they describe.