The passage of the sun across the sky — dawn, day, dusk, night — drives the clock of life. Some species wake with the sun and sleep with the moon. Others do the opposite, and a few keep odd hours. These naturally driven, 24-hour biological cycles are known as circadian rhythms, and they do more than cue bedtime: They regulate hormones, metabolism, DNA repair, and more. When life falls out of sync, there can be dire consequences for health, reproduction, and survival.

Lacking watches, many species keep time using an internal system — a set of interacting genes and their protein products that effectively keeps track of a 24-hour period — that is calibrated by sunlight. This kind of circadian clock is widespread, found even in single-celled algae, which suggests that biological timekeeping evolved billions of years ago. Across animals, most species have the same genetic system, using genes known as CLOCK, BMAL1, and CRY, or recognizable homologues. This form of biological clock mechanism appears even in ancient lineages, including sponges and some jellyfish.

But is this the only way to do it? In a pea-size jelly off the coast of Japan, biologists are examining a different kind of timekeeping.

Somewhere over the course of their evolution, the class of hydrozoans — which includes certain kinds of jellyfish, hydras, and colonial siphonophores such as the Portuguese man-of-war — lost the genes that operate circadian clocks in the rest of the animal kingdom. Yet a newly discovered hydrozoan jellyfish species has a mysterious circadian clock that regularly tracks 20-hour periods, suggesting that its mechanism evolved independently. The findings, published in PLOS Biology in January 2026, push the limits of what chronobiologists consider “circadian.”

“We’ve wondered, do jellyfish have real clocks?” said Ann Tarrant, who studies circadian rhythms in sea anemones at the Woods Hole Oceanographic Institution and was not involved in the research. “This study is really exciting because it shows a clock in this animal that’s lost some of these genes that we think are essential for circadian regulation in most other animals.”

The clock found in this jellyfish, a new species to science, is unusual not only because it tracks 20 hours, instead of Earth’s 24-hour day length, but also because it seems to be paired with a molecular timer that counts down from sunrise until it’s time for the jellyfish to spawn. This surprising mechanism suggests that scientists may be overlooking unconventional clocks across the tree of life.

“Systems like this might be much more widespread, and we are not looking, because we only look at these genetic components, [the animal CLOCK genes],” said Ezio Rosato, a chronobiologist at the University of Leicester who penned a scientific commentary about the work. “You could make a clock with any molecular mechanism. All you need is a series of reactions which are organized in a certain way.”

Once a quarter, Ryusaku Deguchi brings his students at Miyagi University of Education to Izushima, a 1-square-mile island in Sendai Bay along Japan’s northeastern coast. There, thousands of translucent orbs smaller than peas bob in the water column below the fishing dock. He and his students collect these jellyfish specimens, representing more than a dozen species, and rear them in the lab to study their reproductive cycles.