Night on the TARDIS. Technically, there was no such thing, the Doctor reminded himself. He often made a point of educating his companions about this fact. Day and night are planetary concepts, related to one's relationship to an in-this-case-nonexistent sun and the rotation of a celestial body.

In the Vortex, he would tell them, trying to keep the hint of superiority out of his voice and failing utterly, there is no day, and no night. In space, any such constructs are imaginary at best.

And yet, has that ever stopped even one of his companions from wasting nearly a third of their life sleeping? Well no. Each new companion struggled, whether they realized it or not, with coming to terms with living away from a circadian rhythm and a cycle of lights and darks, went far too long without sleep, and eventually settled in to a cycle that very closely mimicked home – needing sleep roughly every 18 to 20 hours. The Doctor's mind boggled at the sheer amount of downtime.

No wonder humans hadn't made it any further than the moon.

The shorter-term struggle was no less pronounced. Eager or shy, intrepid or intimidated, if there was one thing the Doctor had learned over the years, it was that nearly all of his companions had difficulty adjusting to their first few nights on the TARDIS. Oh, most of them were thrilled to be there, he couldn't deny that. But almost no one settled right in and went to sleep on that first night. There was something about slowing down from the initial rush of adventure that seemed to bring crashing home to each person in their own way that they were far from home, literally millions of miles away in some cases, lost in time and space. Night time tended to make most humans feel their vulnerability in new ways.

He'd come to expect a few problems. They just each expressed it in their own way.