Melatonin, a hormone available in over-the-counter supplements and popped freely by many frequent air travelers, may affect the sex glands, U.S. and Japanese researchers reported on Monday.

Tests on Japanese quail showed the hormone regulates a sexual pathway believed to be involved in seasonal breeding patterns.

It is likely to affect human gonads as well, the researchers said.

“It really amazes me that melatonin is available in any pharmacy,” said biologist George Bentley of the University of Washington and the University of California, Berkeley.

“It is a powerful hormone, and yet people don’t realize that it’s as ‘powerful’ as any steroid. I’m sure that many people who take it wouldn’t take steroids so glibly,” added Bentley, who worked on the study.

“It could have a multitude of effects on the underlying physiology of an organism, but we know so little about how it interacts with other hormone systems.”

Writing in the Proceedings of the National Academy of Sciences, Bentley and colleagues at Hiroshima University in Japan said they were studying melatonin’s effects on a brain hormone called gonadotropin-inhibitory hormone or GnIH.

They removed all melatonin-producing organs from the birds - the eyes and the pineal glands - and found GnIH levels fell. When they gave the birds melatonin, levels of GnIH went back up.

This is important because GnIH has been found to have the opposite effect to the key hormone that primes the body for sex - gonadotropin releasing hormone or GnRH. In birds, switching off GnRH causes the gonads - the testes and ovary - to shrink as part of the birds’ yearly cycle.

In humans, GnRH brings on puberty.

Melatonin regulates the sleep-wake cycle in many animals, including humans. It is produced at night by the pineal gland at the base of the brain, bringing on drowsiness.

Light causes levels to drop, and melatonin pills are often used by travelers to help them sleep through jet lag and by shift workers who have trouble sleeping.

Bentley said the team has found a gene for a hormone like GnIH in hamsters, and he found evidence of a GnIH gene in the map of the human genome.

Such a hormone would be important for many species, he said.

“Reproduction is energetically costly. It takes its toll,” Bentley said in a telephone interview.

“So that is why a lot of animals breed seasonally. They can only afford to do it at certain times of year.”