The Great Sleep happens twice in Jack's life. The first time is when he's gotten the full story of Cass Mastern, and the second is when he realizes his relationship with Lois is a sham. In both cases Jack doesn't know how to deal with the new knowledge that has crept into his brain. So he shuts down until he can figure out how to deal with it. In the case of Lois, he tells us exactly what happens. He begins to realize that she's a human being, and not simply a beautiful sex object.
Jack certainly doesn't dehumanize all women. Lucy, Sadie, his mother, and Anne are all flesh and blood creatures with beating hearts. So, his dehumanization of Lois probably is in large part due to his deep love for Anne, and the pain he still feels at the failure of their relationship. If Lois isn't human, and therefore unlovable, Jack can't get hurt, and can't hurt Lois. When he realized he's just using her, he has to sleep his way through the situation until he decides to leave her.
Jack isn't sure or doesn't tell us exactly why the Cass Mastern story makes him crawl under the covers of the Great Sleep. We think it also has to do with dehumanization. Jack already knows that slavery dehumanized both the victim and victimizer, but the story of Cass suggests that slavery so deeply taints humanity, America, and the South, that there is no hope. This might also explain why he falls into the second Great Sleep when he realizes he's dehumanized Lois. He may have felt that his dehumanization of her was like an extension of the mindset that allowed people to justify the enslavement of other human beings.