Paraphrase

Hello:

I am reading a short story by N.Gordimer. I read the following passage:

"I was staring at the door, making it out in my mind rather than seeing it, in the dark. I lay quite still —a victim already— but the arrhythmia of my heart was fleeing, knocking this way and that against its body cage. How finely tunes the senses are, just out of rest, sleep! I could never listen intently as that in the distractions of the day; I was reading every faintest sound, identifying and classifying its possible threat."

I have a couple of questions:

1-I find some troubles in understanding the sentence marked in red. More specifically, the part after the coma "just out of rest, sleep". Could it be paraphrased like this?: "How finely tunes the senses are when you are just out of rest, when you are just out of sleep"? Does it mean you are hypersensitive when you have not had enough sleep?

2-In the phrase marked in italics "the arrythmia...", I understand the head of the subject is ARRYTHMIA and OF MY HEART a postmodifier. Then how can the arrythmia flee and knock this way and that against its body cage? The arrhythmia does not have a body cage! Could you please tell me what I am missing here?

Thank you very much.

Julia