You are of course refering to the second half of this chapter, when Jane tries to sleep but is interrupted by strange sounds and events that prevent her from gaining a quiet night's repose. The change of tone is achieved through a number of different techniques, but above all you will want to analyse the role of diction in this change of tone as the mood of quiet peacefulness turns into something much more sinister and dark. Consider the following paragraph:
There was a demoniac laugh--low, suppressed, and deep--uttered, as it seemed, at the very key-hole of my chamber door. The head of my bed was near the door, and I thought at first the goblin-laughter stood at my bedside--or rather, crouched by my pillow: but I rose, looked round, and could see nothing; while, as I still gazed, the unnatural sound was reiterated: and I knew it came from behind the panels.
Consider the use of the adjective "demoniac" which is used to describe the laugh. This, in addition to the way that the sound seems to emerge from just by Jane Eyre's room and the reference to supernatural creatures such as goblins also help to convey the dramatic change in tone. Such Gothic elements, combined with the lack of vision that Jane has, and then the repetition of the sound, preys on our mind just as it does on Jane's mind, opening us up to terror and fascinated horror.