What is creating these rhythmic patterns?
Rhythms have been found to be a result of both biological and environmental factors. Biological factors include influences from hormonal and neural modulation from the suprachiasmatic nucleus of the brain, as well as effects of local signals in nearly every cell and organ. These neuronal and molecular signals (sometimes called "clocks" or synchronizers) control the timing of genetic transcription, of important chemical reactions, or other biological activity or levels of bio-molecules. They promote or inhibit biological activity. (To understand how chemicals can signal each other, and create cycles see the experiment below.)
Environmental factors are largely comprised of the light/dark cycle, and the effect of eating on the organism. The light/dark cycle is associated with sleeping and awake states, and is used to orchestrate a plethora of bodily functions that benefit from alignment with light/dark. Over millennia the organisms have adapted to take advantages of this 24 hour cycle. Eating is a factor because it triggers a cascade of biological events related to digestion throughout the body, impacting stomach, intestines, kidney, bladder, liver, bloodstream etc., as well as damping systems related to flight and fight, and bodily repair. When resources are directed to the digestion and the parasympathetic system, they are redirected away from the sympathetic system, or the fight and flight system. Thus many systems are impacted by eating.
Cycles occur in all living things, as well as isolated tissues and chemicals.
An enlightening demonstration of how cycles occur through simple chemical reactions involves six readily available chemicals: hydrogen peroxide, potassium iodate, sulfuric acid, mallonic acid, soluble starch, and manganese sulfate. When these chemicals are combined in the correct amounts, the solution turns from amber to blue-black to colorless in cycles of about 15 seconds, for several minutes. Details can be found on page 83 of the book, or page 91 of the pdf.