In 2025, Jamie Perce with the Western Slope Dark Sky Coalition presented an excellent series on circadian rhythm. The importance of this topic is prompting a deeper dive in 2026 with a five part series.
Long before the first sunrise illuminated human eyes, life on Earth had already invented the clock. Your circadian rhythm is an internal timekeeping system that evolved over millions of years to synchronize your body with Earth’s rotation.
What makes these rhythms remarkable is that they’re endogenous—originating within the organism itself. At the cellular level, they operate through precise genetic feedback loops involving core clock genes that regulate their own production in roughly 24-hour cycles.
Here’s where it gets fascinating: these circadian clocks exist throughout nature—in animals, plants, fungi, and even cyanobacteria—yet they evolved independently. Different organisms, facing identical selective pressures from Earth’s relentless rotation, independently invented similar solutions using completely different molecular toolkits. The advantage is clear: organisms that anticipate daily changes outcompete those that merely react.
Consider cyanobacteria, ancient photosynthetic organisms that existed over three billion years ago. They needed sunlight for energy yet also required nitrogen fixation—a process disrupted by oxygen. Their solution? A biological clock that separated these incompatible processes across the day-night cycle.
These same cyanobacteria triggered Earth’s Great Oxidation Event 2.4 billion years ago. The consequences were catastrophic. Earth’s early atmosphere had been rich in methane, a potent greenhouse gas that kept the planet warm despite our sun being significantly dimmer than today. When rising oxygen reacted with this methane, converting it to the much weaker greenhouse gas carbon dioxide, global temperatures plummeted to minus 50 degrees Celsius. As ice spread, it reflected more sunlight, creating a feedback loop that froze the entire planet creating Earth’s first “Snowball Earth” event, also known as Huronian Glaciation.
Life’s own innovation had fundamentally altered atmospheric composition, triggering global catastrophe. Yet through it all, those circadian clocks kept ticking.
The evolution of circadian rhythm shows that life doesn’t just adapt to its environment—it internalizes it, building representations of planetary motion into cellular machinery. For billions of years, the light-dark cycle shaped life at the most fundamental level.
So what’s the take home message? We evolved in synchrony with our planet, deeply adapted to the 24-hour rotation of the Earth. For billions of years, the light-dark cycle shaped life at the most fundamental level. Then, in the late 1800s, we invented artificial light at night—flipping the switch and suddenly disrupting billions of years of evolutionary synchrony.
In upcoming episodes, we will explore how this disruption affects our health, environment, and connection to the night sky.
Music written and produced by Kenny Mihelich. Western Slope Skies is produced by the Colorado Mesa University Astronomy Club, the Western Slope Dark Sky Coalition and KVNF Community Radio. This feature was written and shared by Dr. Kate Fedack.