What is the reason the narrator is afraid?

The narrator is afraid because because he/she was awakened by something he/she could not distinguish. Unable to go back to sleep..... all sorts of things pass through the narrator's mind.

And then last night I woke up—or rather was awakened without knowing what had roused me. A voice in the echo-chamber of the subconscious?

A sound.

A creaking of the kind made by the weight carried by one foot after another along a wooden floor. I listened. I felt the apertures of my ears distend with concentration. Again: the creaking. I was waiting for it; waiting to hear if it indicated that feet were moving from room to room, coming up the passage—to my door. I have no burglar bars, no gun under the pillow, but I have the same fears as people who do take these precautions, and my windowpanes are thin as rime, could shatter like a wineglass.