Every time this comes up, I recommend "Internal Time" by Till Roenneburg. It is wonderfully written, and it demolishes this concept quite thoroughly. It really

Every time this comes up, I recommend "Internal Time" by Till Roenneburg. It is wonderfully written, and it demolishes this concept quite thoroughly. It really is 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.