Earlier quoted context omitted.
Be warned, that book is riddled with scientific and factual errors: https://guzey.com/books/why-we-sleep/
Very interesting writing, I wonder if the people sleeping just 6 hours (who are in the best condition acc. to a later explanation) wouldn't be because they have a cause in their life? They have a purpose that is why they get up early and motivated enough to work on it. For people with more sleep might be less motivated in average? Interesting topic nevertheless.
When I wake up, even if it's 4.5hrs, my brain is typically going a mile-a-minute. There's so much I want to accomplish, it's hard to be mindful and go back to sleep (so I rarely do, especially if it's 'only' 6hrs vs. the 7.5 I seem to need).
I looked back at my sleep logs, and I've slept longer than 7.5 only a handful of times in 2020, and it's always correlated with several nights of 1.5, 3, or 4.5 hrs.