Three short lines set up a surprising comparison: the woods at night are as busy as the inside of a clock. We usually think of night in a forest as quiet and still, but the speaker suggests the opposite. In the dark, small creatures, rustling leaves, dripping water and shifting branches keep working away, much as hidden gears keep ticking out of sight.
The last line, "when I am asleep," adds a twist. The speaker can't actually witness this activity, so the busyness is something imagined or trusted. It exists whether or not anyone is there to notice it. That gives the poem a quiet sense of wonder, and maybe a touch of humility, since the world carries on perfectly well without us.
The mood is calm and curious rather than eerie. The clock image makes the night feel orderly and purposeful, not threatening. Even the poem's brevity fits: like a clock's face, it shows very little while hinting at a great deal of movement underneath.