High levels of melatonin can cause ovaries and testes to shrink in seasonally breeding animals, leading to infertility.

“Long ago when I was at UC, Berkeley as a master's student, I was working on neuroendocrinology, and we were working on this hamster species of seasonal breeders. And basically when days are long, which inhibits melatonin, these little Siberian hamsters as they're called, have testes about the size of, sort of typical table grapes, although that's a weird way to put it. When days get shorter and the melatonin signal gets longer because light inhibits melatonin, days get shorter, melatonin gets longer. Those same hamsters would have testes that would involute to the size of about a grain of rice.”