The Doctor thought a moment -- the Winchesters had been fairly forthcoming about their work - unconventional as it might be, and that gave him the sense that perhaps there was a certain element of trust there - something not to be taken for granted.
"Travel - time and space. World, galaxy, universe; that whole sort of thing," he said quite matter of factly. Nothing in his tone indicated a joke, and certainly nothing indicated he was bragging either. It was as if he were being asked what color shirt he was wearing.
The Doctor saw Sam's grin fade, and he felt his own slide off a bit as well. As wearied as he was, it was hard not to think that sleeping might be a fair bit dangerous. "Jack's around here someplace," the Doctor remarked. "He doesn't actually sleep, so we should be alright to at least try and get a bit of rest." The Doctor would have to remember to thank Jack later for this.