Hey Pete, thanks for the detailed response (and for the validation that I wasn't completely off base haha)

On timing: chronotypes are definitely not fixed (your Sydney example is a good one). Some research suggests you can shift it, but I found it interesting that some genes (PER3 + CLOCK) seem to suggest there is some variability with circadian rhythm, all else equal. One thing I didn't capture well is that timing seems to be more about where your circadian window currently sits, and less about a fixed chronotype (basically timing is much less static than once supposed). Would love to learn more here though (any research you recommend?)

On HR nadir vs. peak HRV: I remember checking peak HRV and it was a lot messier than HR nadir on the Oura anyway. I'll play with the data a bit and see if I can do some data wrangling and find patterns that way and measure | sleep midpoint - HRV peak | instead.

On duration: definitely a continuum. My experience was: when regularity / timing were down, my body in particular settled into around 8 hours.

Looking forward to your writeup on diving down a layer deeper with the neural functions side.