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.
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.
Bonnie - I have always told my clients before taking melatonin (which I never suggest long-term) to test their melatonin hormone level first. If you are not deficient in melatonin, it may be harmful to take. Like administering any hormone, it may create an imbalance of other hormone levels.
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