Why couldn't Susan and Lucy sleep?

Excerpt from jane eyre charlotte bronte 2 i was glad of it: i never liked long walks, especially on chilly afternoons: dreadful to me was the coming home in the raw twilight, with nipped fingers and toes, and a heart saddened by the chidings of bessie, the nurse, and humbled by the consciousness of my physical inferiority to eliza, john, and georgiana reed.