Something to ponder; every living thing on earth is subject to the circadian (24 hour) rhythm. It is understandable why animals and plants need to be awake in daylight hours. Less so for fish that for thousands of generations have lived in underground rivers and have over the millenia lost the ability to even sense light. Even less so for bacteria. But still, all of us have this endogenous clock keeping time within us, keeping time with the sun.
In the 1930s, a scientist, Nathaniel Kleitman and a colleague attempted to change their body clocks. They spent a month in a cave, 140 feet underground with no natural light and a constant temperature of 54 °F. They used lanterns to regulate their "daylight". Each day they slept for 9 hours, worked for 10 and rested for another 9. They measured the rhythm of their body temperatures but could not adjust either that or themselves to the 28-hour cycle, it stubbornly remained at 24 hours no matter what.
One of the most intractible sleep disorders is that where the person's body clock does not conform to the universal circadian rhythm. The example given in the book is of a boy whose cycle shifts by an hour a day. For a few days a month he sleeps and is awake and working efficiently at the same time as his schoolmates. But nothing the doctor, the author could do, or any medication, could stop his natural wanting to sleep and wanting to wake to a 24 hour rhythm. The disorder made education very difficult, but as a man, he can work for himself and choose his own hours.