While Enfield's account of Hyde's brutal attack on the young girl certainly disturbs Utterson, it's not the images that keep him awake. He is more troubled by the *mystery* surrounding Hyde and his connection to the seemingly respectable Dr. Jekyll.
Here's why Utterson can't sleep:
* Intrigue and Curiosity: Utterson is a lawyer, a man of logic and reason. He's fascinated by the puzzle of Hyde and his connection to Jekyll. He tries to understand the situation, to piece together the clues and find the truth. This intellectual puzzle keeps him restless.
* Concern for Jekyll: Utterson cares deeply for his friend Jekyll and is worried about the possibility of a dark secret that might be harming him. The fact that Jekyll seems to be hiding something creates a sense of unease and concern, keeping him awake.
* Moral Duty: As a lawyer and a man of high morals, Utterson feels a sense of responsibility to investigate the situation and protect Jekyll from any potential harm. This sense of duty keeps him preoccupied and unable to relax.
The images of the attack are disturbing, but Utterson's inability to sleep stems from the mystery surrounding Hyde and his connection to Jekyll, and his desire to understand and help his friend. It's the psychological puzzle, not the graphic imagery, that keeps him awake.