A is for Apple-grass, Accident, and Art
It was an accident, the first time they slept together. Purely sleeping, mind you, nothing more. Things had finally calmed down after the attempted Sycorax takeover, but the Doctor was running on adrenaline. Not that he'd admit that, after all, as he was fond of reminding his companions, Time Lords were physiologically advanced compared to humans, and needed very little sleep.
Post-regeneration, adrenaline, however, was still adrenaline, and prone to disappearing at inopportune moments. Like while a 900-odd year old Time Lord was lying with his companion on a bed made of his duster, with apple scent wafting off the vivid-green grass and the sound of waves from the nearby bay.
Rose, on the other hand, as a companion to said Time Lord knew to get sleep when she could. She had learned the hard way that letting her sleep was not an option when adventure was to be had. And, of course, there were much worse places, such as jails, harem rooms, dumpsters, to sleep. So Rose, curling up like a cat had dropped off as soon as the Doctor had stopped his babbling (which took a while) to stare off into the distance, as overly dramatic Time Lords were prone to do.
And so it was that when the Doctor next looked down, his breath caught in his throat. Not in the cliched "My beautiful, beloved Rose, I am caught breathless by your innocence and trust" way, but in a simple, surprised way. She looked like a work of art, like a photo of herself, stress gone from her face and body, completely relaxed.
Smiling, the Doctor lay behind her, meaning to wait in comfort until she woke up again, exploring be damned (just this once, after all, he still didn't believe that humans needed a whole eight hours sleep to function). And it was then that his adrenaline wore off, leaving him too tired to even alleviate his tiredness biologically. Instead he almost immediately conked out.
When they woke up, another adventure would be ahead, but for right now, the two travelers slept next to each other, in a beautiful landscape, looking for all the world like a 21st century romantic movie. Well, except for the flying cars.