The "early wakeup time makes people groggy" story always looked puzzling to me. I mean, when DST kicks in everyone adjusts in day or two. What prevents one from setting up their own personal DST and getting up earlier still? Certainly not biology.
Summary of the book as relates here though: People really do have internal clocks and preferred awake/sleep cycles. These internal clocks vary between individuals, and are there is most likely a genetic component. These internal clocks vary with age- in particular, teenagers and young adults clocks are generally shifted much later than kids or older adults. Shifts to people's schedules (such as DST) that cause them to leave their preferred sleep/awake cycle mitigate with time, but do not fully go away.
Outside that book, there are plenty of studies showing that a non-standard hours shift work is biologically harmful.