Humans have an internal clock of around 24.2 hours and varies from something like 24-25 (not normally distributed...the longer day tail is much longer). Sunlight is the primary driver of it through our SCN , and since sunlight is on a 24 hour cycle, we adjust to that.

Where exactly we set our sleep time vs sunset varies on how close our internal clock is to 24 hours with longer clocks being night people and shorter being morning people.

There have been studies on how far humans can adapt. The nicest I'm aware of are from Richard Kronauer's team and tested 23.5 and 24.6 hour days with different light exposures:

https://pubmed.ncbi.nlm.nih.gov/17684566/

We can maybe adjust to day lengths outside of that. I was a researcher on studies to determine what submarine schedules were viable for the Navy, and 18 hour days were outside of the range of what humans could adapt to.