Be warned, that book is riddled with scientific and factual errors: https://guzey.com/books/why-we-sleep/
https://sleepdiplomat.wordpress.com/2019/12/19/why-we-sleep-...
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.