There’s no clear set of genes that causes delayed sleep. You definitely have those rare mutations of core clock genes which causes their circadian cycle to have distinct properties, one of the outputs of which is a natural sleep preference that’s out of phase with typical societal rhythms, but is still a normal amount of sleep, and its consolidated and restful etc.

But the clock itself has a population wide variation in its actual period. It varies between 23.8 hours and 24.6 hours, and everyone far from the 24 hour ideal pays a tax. The clock either has to phase advance or phase delay to synchronize, and over time that has all kinds of impacts, including on sleep, but also memory and learning, stress, attention…

The whole system is clocked. Every cell in our body runs a 24 hour cycle, driven by the transcription, translation, inhibition and breakdown of the core clock proteins. They’re in a negative feedback loop, with a time delay, and all kinds of mutations can shift the overall period of the clock. If it’s of a core clock protein, the effect will be strong, but other genes cycle or have their functions in a cycle due to the clock, and feed back into the clock, so small mutations can accumulate to cause circadian period variation.

It’s also why you see circadian phenotypes in all kinds of diseases. Definitely needs more research, but the research is also hard. You need to observe the body in high resolution over many time points to start making sense of all this, and that’s expensive and exhausting. We’ll get there, though.