Four short lines, and a quiet surprise. The speaker says that when night arrives, they can only shake. Most readers will expect the next line to explain it as fear, since darkness and trembling usually go together. The poem turns that expectation around: it isn't fear at all. What comes alive in them is their "true self."
That twist is the heart of it. The trembling seems to be a physical sign of something powerful stirring, maybe excitement, release, or the relief of no longer holding back. The poem suggests that daytime may require a kind of mask, and that night strips it away. The word "only" hints that the speaker has no control here; the shaking simply happens.
The mood is intimate and a little mysterious. It feels secretive and charged, but not frightening. Night becomes a place of honesty rather than danger, where the speaker finally feels like themselves.