Four short lines, and almost all of them circle one problem: the speaker can't sleep. It's one in the morning, and the opening question, "How to sleep?", sounds less like a puzzle than a tired plea. Nothing about it suggests an answer is coming.
The second line is the strangest. Ash pouring into the mouth is a dreamlike, unsettling image. It might point to a feeling of choking or burnout, or to the dry, bitter taste of exhaustion. The poem doesn't explain it, and that gap adds to the uneasy mood.
The last two lines show a reversed routine. The speaker falls asleep on the table after dinner, when sleep isn't wanted, and then at night "walk[s] up and down in bed," pacing in the one place meant for rest. Sleep and wakefulness have swapped places. The mood is weary, restless and a little absurd, as though the speaker is stuck in a loop and too tired to fight it.