The speaker is afraid of falling asleep, and the title suggests why: sleep is pictured as a werewolf, a creature that takes over and feeds. The poem sets up a small, strange scene. There is "a bed in my mind" and "a hat on the house," as if the speaker's head and home are being prepared for a guest, or marked as already claimed. Then comes the plain admission: "I fear a good sleep."
The last lines turn sinister. Sleep "goes to take / possession and / to sup," meaning it arrives to seize the speaker and to eat, much as a beast might. Judging by the imagery, the fear may be about losing control, or about what happens to a mind when it gives in to unconsciousness.
The mood is uneasy but quiet. The short lines and sparse words make it feel like a whispered confession, a tiny dark story told in the moments before the lights go out.