A restless sleeper wakes, or half-wakes, to the sound of his own heart pounding too loudly. In that groggy state, his "night body" is ready to believe something frightening: that it is lost, and that he himself is a foreign land, a "strange country" it can't find its way through. The heart's racket seems to turn his own body into unfamiliar territory.
The last lines widen the feeling. "A hundred ears" suggests a body straining to listen in the dark, picking up every small sound. Yet each day he seems to be "losing ground" to the "void / in my bed." The title tells us what that void is: someone's absence. The poem never says who is gone, but the loneliness is clear. The mood is anxious, hushed and sad, with the blues of the title carried in the quiet. Short lines and little punctuation make the poem feel like a thought drifting through a sleepless night.