The rhythm inside us

The 24-hour clock that runs our internal systems is called a circadian rhythm. Circadian rhythms are complicated, so to understand how they work, scientists often look at how daily cycles show up in very simple organisms.

The most basic rhythms appear in single-celled organisms that only reproduce in the dark to protect their genetic material from the harmful UV rays in sunlight. The triggers of this circadian rhythm—and those that have evolved from it—lie in the genes. Observing activity at the genetic level can be difficult, but, thanks to some high-tech tricks, it’s not impossible.

Light is the essential trigger of many biological rhythms. Sunlight entering through our eyes resets our internal clocks each day.

Photo courtesy Simon Zirkunow via