Because the human "clock" is probably very close (24:11?) to 24 hours, I (and, probably, you) should not think we are "naturally" wanting to stay up later and later and would be happier on a planet with 25-hour (or longer) days. My working conjecture is that it's artificial light. "The human clock consists of a cluster of nerve cells (the suprachiasmatic nucleus) barely a hundredth of an inch in size, located deep in the brain and connected to the eyes’ optic nerves."

A well-known related theory is that blue light has the greatest effect. I'm currently (self-)experimenting with reddening my home desktop LCDs as much as possible in hardware. However, effects of this treatment on when I actually fall asleep have so far been non-obvious. I would need to more carefully collect data and experiment with undoing the reddening for a while to be sure, but if the effect is small enough that I can't perceive it directly, then the effect is not worth the hassle of avoiding blue light. Relatedly, an N=12 study found that, "as expected, only the blue light reduced nocturnal melatonin. In contrast, both blue and red lights affected cortisol levels."

On the other hand, reddened screens at night are easier on my eyes, whether they help me sleep or not.