There is a base circadian period which is roughly the length of a day, but it varies between individuals. If you put people in caves with no light at all, they'

There is a base circadian period which is roughly the length of a day, but it varies between individuals. If you put people in caves with no light at all, they'll still naturally fall into a sleep/wake cycle around 24 hours (it tends to be slightly longer). Specifically it comes from your SCN https://en.m.wikipedia.org/wiki/Suprachiasmatic_nucleus https://en.m.wikipedia.org/wiki/Suprachiasmatic_nucleus The answer is it's evolved and some kind of complex cellular feedback mechanism, but Wikipedia is a bit light on details. Apparently this clock is also fanned out in some way to other parts of the brain/body which also follow a daily cycle. https://www.sciencedirect.com/science/article/pii/B9780444594273000290 https://www.sciencedirect.com/science/article/pii/B978044459... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2735866/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2735866/ Light isn't mandatory, but the SCN will synchronise to daily light patterns (eg when you travel). The SCN is also physically located close to the optic nerve. I think this is why the underlying rhythm is a slightly longer than a day, because it allows the body to reset it to 24 hours. If the period was shorter, even slightly less than 24 hours, maybe you'd have problems when the clock restarted before the day finished and the error would propagate over time (you'd keep shifting the cycle).