Never to Sleep — Rachel Vincent

Don't Close Your Eyes.

Sophie Cavanaugh is not going to let her freak of a cousin's strange psychiatric condition ruin high school for them both. Not after all the work she's put into cultivating the right look, and friends, and reputation. But then, Sophie sees something so frightening she lets out a blood-curdling scream -- and finds herself stuck in a bizarre parallel world where nothing is safe and deadly creatures lurk just out of sight, waiting for her to close her eyes and sleep...forever.

Could this world be real? Or does insanity run in the family...?