Four short lines, and a strange request: the speaker asks to be put to sleep, emptied of feeling, and then turned into writing. The title helps. A somnambulist is a sleepwalker, someone who moves and acts without being awake to it, and that seems to be the state the speaker wants to be in while the poem gets made.
The tone is hushed and a little surrendering. The speaker gives orders ("Lock me," "discharge me," "set me"), yet each one hands control away. Sleep locks, numbness releases, and writing fixes. There is something almost passive about it, as if the speaker can only exist by letting something else act on them.
The last line, "and so to be," is the turning point. Being doesn't come from waking or feeling. It comes from being set down on the page. Judging by the wording, the poem suggests that writing happens best when the conscious mind steps aside, and that the self becomes real only once it is written. The mood is calm, odd, and quietly dreamlike.