Endogenous melatonin can suppress the maturation of gonads, including reducing sperm production, testosterone production, and egg maturation.

“Melatonin is also involved in maturation of the gonads during puberty, the ovaries and the testes. Although there, the effects of melatonin tend to be suppressive on maturation of the ovaries and testes, meaning high levels of melatonin tend to reduce testicle volume and reduce certain functions within the testes, including sperm production and testosterone production. And within the ovaries, melatonin can suppress the maturation of eggs, et cetera. Now, I don't want anyone to get scared if you've been taking melatonin. Most of the effects of melatonin on those functions are reversible, but I should point out that one of the reasons why children don't go into puberty until a particular age is that young children tend to have chronically high endogenous melatonin.”