Not sure which part of the book you read, but Dr. Walker has written in that book and stated in many talks and interviews that there are wide varieties in what the average person needs for sleep. The sweet spot for the majority of people is about 7 hours, more or less.

Young children may need up to as much as 10 or more hours sleep; people over 60 may do fine on 6 1/2 to 7.

As far as waking up at night, the solution is simple. Don't sweat it. the "official" treatment in cognitive behavior therapy for insomnia is called (terrible name) "stimulus control."

This means if you wake up, you must get out of bed otherwise you'll associated the bed with being stressed an unable to sleep.

Thankfully, this has been modified in recent years as research has found some people thrive on the practice of deep relaxation and/or breathing, meditation, etc when they wake up in the middle of the night, and thus do not have sleep problems as a result.

Furthermore, the "wake back to bed" method of inducing lucid dreams actually calls for you to intentionally wake up in the middle of the night, recall the previous dream, and vividly imagine yourself going back into the dream, aware you're dreaming.

TLDR: There are many variations in what works for people in terms of sleep; the 8-10 hour cycle is good for some but not most.