Circumdiem; or, "About One Day"

The Online Etymology Dictionary explains how the term "circadian" was "coined 1959 by German-born biologist Franz Halberg, from Latin circa "about" (alternative form of circum "round about;" see circum-) + diem, accusative singular of dies "day" (from PIE root *dyeu-"to shine"). The original use is in circadian rhythm." (source)

Think of the circadian rhythm as a pre-installed, ready-made clock for every living creature, found in that part of the brain that controls pretty much everything we can't control: body temperature, fluids, hormone production, organ function. Interestingly, that part of the brain is connected to the eye, the sensor that detects light. Our chemistry changes when it gets dark or light, which means artificial light at night does not help the clock. Our circadian rhythm also changes with the seasons, which may explain why we "slow down" and are inclined to stay under the covers just a little bit longer. Studies show that strength and pain tolerance is highest in the afternoon, which is ideal for mid-day PT. 

Marcus Aurelius did not have the science but in a note to himself about staying in bed he wrote, "we have to sleep sometime . . . but nature set a limit on that--as it did on eating and drinking. And you're over the limit. You've had more than enough of that. But not of working. There you're still below your quota." (Meditations, 5.1)

We are made to work. Rest cannot be fully understood apart from work so when one no longers gives attention to the circadian rhythm, to "the beat of our own drum", we do more harm than good: resting when we should be working or working when we should be resting. Awake when we should be sleeping, missing a meal when we should be eating, etc. Our bodies are designed to tell what to do and when to do it--we are too distracted to listen and so make life harder for ourselves and wear out.