Based on Season 1, Episode 20 "Dead Man's Blood"

John looked over the top of Elkin's diary to where his sons slept on the beds in front of him. They were fitful and murmuring – Sam would be uneasy he knew, his younger son still hadn't forgiven him, but it was peculiar for Dean to be so. Dean could sleep anywhere.

He paused his research to watch them a moment. He would never stop blaming himself for forcing them into this life, but it was a responsibility they were holding well.

Dean turned over to his left and John smiled when Sam did the same instinctively. But his smiles soon grew to frowns when the patterns were repeated. Sam groaned – perhaps nightmares gripping him, who could blame the boy – and Dean reached across his empty bed as if to calm him. Sam leant back into empty space and fell back into a sound sleep.

John could hardly be surprised – unnatural was part of the job, but as he dipped his head back into his research, he knew the twin beds were just for his benefit.