Lying awake at night, the speaker can hear their own heart, and what they hear isn't comforting. The poem is only three short lines, but it captures a very particular kind of loneliness: the moment when everything else goes quiet and the body's own noise becomes impossible to ignore.
The word "heave" is the key choice. A heartbeat is usually described as a soft thump or a gentle rhythm, but a heave suggests effort, as if the heart is straining to keep going. That makes the sound feel heavy and tired. Calling it "a sad noise" turns a physical fact into an emotional one, so the speaker seems to be hearing their own sorrow or worry as much as their pulse.
The setting adds to this. "At night in bed" is when people are alone with their thoughts, and the poem's brevity leaves a lot of silence around those few words. The mood is quiet, vulnerable, and a little anxious, with a sense of being awake and aware of being alive.