[enknamel]

Can't biological clocks change? Or are there really populations of people who have a truly different clock? Or is it a result of their environment?

Personally I've been a night owl and a morning person alternating between the two throughout my life by choice.

[Eric_WVGG]

Sure. Mine, and I think many others, changed precisely when I went through puberty. Early riser before, late riser after and ever since.

My grades (predictably) crashed early in high school, didn't start to recover until I discovered coffee. But that couldn't be found in my Utah high school, of course, so I either had to sneak a cup in or find someplace to "joe up" during lunch.

Additionally, six years ago I moved into an apartment with a huge east-facing window. Now I'm a natural morning person again — during the summer months.

[digitaltrees]

There is an actual hormone release cycle and people have different timing. You can affect it somewhat with light therapy but only by an hour or two.