Carl can see that.

"All right," he says.

"I'm going to guess you've not been sleeping either." He pauses a moment. "Would you want me t'take a watch here on the couch while you got a bit?"

He isn't implying that Jack needs someone to fight off the nightmares - he just knows that sometimes it's easier for men like them to sleep if they know that they've got someone posted on sentry duty.