Sing Me to Sleep by R.M. Virtues

Penelope has lost so much in six months.

Her father, her sense of security, her ability — and willingness— to sleep. Her family is most concerned about the latter at the moment, and when her sister Jenna leaves her alone in their shared home for the first time since The Incident, Penelope finally accepts some help. The only problem? There are side effects. Well, one: sleep paralysis.

And the demon who came for her fear.

Fear is the only thing that matters to the demons of the Somni Dae, and as long as they deliver it back to Hell, they are free to do as they please. Except enter the waking world of course, but Acheron never minded that. He has everything he needs right here, and in thousands of years, he has never wanted for anything. Until he stumbles across Penelope’s dreamscape.

Now is not the time to grow attached to a wandering soul, not with Heaven and Hell preparing to wage war. Nor is it the time to grow attached to your sleep paralysis demon, not when getting out of bed is already so hard. But Penelope is finally sleeping, and after Acheron wipes out the last of her fear, they both realize they’re stronger together. And they’re going to need all the strength they can get.