Cullen has dozed off, in the manner of soldiers who can catnap sitting up. As such, when Alistair's breathing changes, Cullen comes fully awake and alert very quickly.

He won't let Alistair have to sit with it longer than he must. Cullen snaps his book shut and puts it on the nightstand, and rests his hand on Alistair's forehead.

(Which leaves him another hand to use if Alistair tries to strike at him. Cullen's got experience.)

"Alistair. Wake up. Now, please. You're safe. You're dreaming."