Our world is full of involuntary rhythm. Our heartbeat. Our breath. The rising and setting of the sun. The seasons of the year. This is rhythm that helps us, but happens without our effort. It's automatic. Some things require us to create rhythm in order to make it better or functional. Music is a perfect example, as is sleep. Rhythm doesn't exist in them by default, and so we need to maintain the rhythm ourselves, manually.
Most people assume their day will have a rhythm, that it is automatic and innate. But in order to get the most done each day, maintain our responsibilities and be productive, we need to take control of our daily rhythms. Wake and sleep at the same time, regardless of what's on our lists. Complete certain daily tasks at the same time each day. Take breaks at the same time.
Rhythm allows us to trust our day and depend on our opportunities to accomplish things. Too much variety breeds confusion and doubt and opportunities for failure. Consistency makes success possible.