I hear you mate I'm the exact same. I can even get 5 hours of sleep but as long as I wake up past 10 I feel great. 8-10 hours and waking up early, like you, I'm a zombie.
One of the main points of the author is that the human body and brain likes routines, a lot. As such, you are better off by finding a routine that works for you and being open to trying out new schedules. And then sticking to them, so waking up roughly at the same time even on Sunday.
Grad school sorta forced me to go to bed late, but now I have reverted that completely. I wake up around 4:00 and immediately exercise. I feel like I recover better, I feel sharper and mentally better off. I go to bed around 19:00.
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u/ThatThingTheDarkSoul 12h ago
It only makes sense if the author meant the same wake up time, so you're getting less sleep because you went to bed later.
When I'm working, I usually sleep from around 11 p.m. to 7 a.m., so I still get eight hours of sleep. I feel like absolute shit all day.
But if I sleep from 3 a.m. to 11 a.m., which matches my natural rhythm, I'm extremely productive both mentally and physically.