This statement is repeated twice, once on page 30 and on this page again. As we know, repetition is used to bring attention to an idea, and often emphasize its importance. This repetition, not only makes me understand what he feels (because there's things that I think about, that give me the same exact feeling), but it also makes understand why he "can never get to sleep at night". He's so haunted by this thought of cannibalism, and how he can be the next victim, that if he thinks about this at night, I understand that it might be impossible to get any sleep, especially after getting a shiver that's twice as intense (because he mentions it twice), running from the top of his head down to the tips of his toes. If we analyze this statement, we'll notice that this shiver starts on his head, which means it also awakens and freezes his mind, and if his mind is frozen (since we know that the brain controls body movements), then his entire body is frozen, hence why he feels that "shiver" through his entire structure.