Eh I don’t think it is that simple. I have always preferred to be a night owl. Also had to get up very early during work days during school and most work years. So now I also get up naturally when it is light. Or at least wake up somewhat alarmed I will be late when even if I do go back to sleep. I go as long as possible in the summer until exhausted I crash for 12 hours or take a nap. In the winter I tend to sleep longer and more… Biological clock???? There is none.