Fish are singing in the San Francisco Bay. There’s no mistaking it. It’s less melodic than a loud drone that begins in the dark of night and starts just before the sun comes up. But why? Scientists think they have an answer.

The fish is Porichthys — or, specifically, the male suitors of this species. They’re also called the plainfin midshipman fish. Females only grunt to show aggression, but males use it try to find a mate, and many have compared their call to a chorus of kazoos or a swarm of bees, according ot a Cornell University statement.

Researchers think they know why they sing at the same time, and it’s because of melatonin, which is trigged by darkness. Melatonin is thought to regulate our internal clocks, and some people take melatonin supplements before bed in the hopes of getting some shut-eye when they can’t sleep.

But for nocturnal vertebrates, scientists haven’t been able to nail down melatonin’s purpose. Now, research publish in the journal Current Biology indicates that melatonin may trigger nighttime behaviors, rather than simply act as a sleepy chemical.

“Our results, together with those of others that also show melatonin’s actions on vastly different timescales, highlight the ability of hormones in general to regulate the output of neural networks in the brain to control distinct components of behavior,” said Andrew Bass, professor of neurobiology and behavior in the College of Arts and Sciences, the paper’s senior author. “In the case of melatonin, one hormone can exert similar or different effects in diurnal vs. nocturnal species depending on the timescale of action, from day-night rhythms to the duration of single calls.”

“Melatonin is an ancient and multifunctional molecule that is found almost ubiquitously in the animal kingdom,” said Ni Feng, Ph.D. ’16, a former graduate student in Bass’ lab who is currently a postdoctoral researcher at Yale, the paper’s first author.

“Similarly, circadian rhythms govern the daily lives of diverse lineages, from plants to animals,” Feng said. “Our study helps cement melatonin as a timing signal for social communication behaviors.”