Meaning of "heed it"?

Hello everyone. I am reading a novel, and I came across this expression. Could you please let me know its meaning?

Suddenly, and more than anything right now, I wanted to call her and hear her raucous sleepy voice and, in hearing it, say to that raucous sleepy voice what I would with difficulty have said to her sparkling daytime voice, things one only mutters in unfettered half-sleep to those who’ll heed it in half-sleep themselves: I don’t care if I wake you up, I want to be with you now, in your bed, under your blanket, in your sweater, life is so very cold tonight, I’ll sleep in the next room if I have to, but I don’t want to be without you, not tonight.

- André Aciman, Eight White Nights, Fourth Night

This is a novel published in the United States of America in 2010. This novel is narrated by the nameless male protagonist who meets Clara at a Christmas party in Manhattan. Three nights after the party, the protagonist is thinking how he wants to call Clara.

In this part, I wonder what the underlined expression means, especially what "it" refers to.

I first thought "it" might refer to "things", but then "it" is singular whereas "things" are plural, so I am confused...

And here, I think "heed" would mean "take notice of" or "pay attention to", though I am not sure.

Thank you very much for your help.