Just before and during your sleep the body releases the hormone melatonin. The better the quality of your sleep, the greater the amount. Melatonin inhibits the conversion of testosterone into estradiol, Thai toxicologists reported in 2014 in Toxicology Letters. Study The researchers, associated with the Chulabhorn Research Institute in Thailand, experimented in test tubes with a mixture of T47D breast cancer cells and healthy adipose fibroblast cells. This combination of cells resembles more like tissue in an organism than just some breast cancer cells. T47D cells are estradiol sensitive. The researchers wanted to know whether small amounts of melatonin impact the growth-promoting effect of estradiol on the cancer cells. Results When the researchers exposed their cells to testosterone in a concentration of 0, 1, 5 and 10 nanomoles, the breast cancer cells grew faster [proliferation]. The higher the concentration of testosterone, the greater the fold change of proliferation. Melatonin blocked that effect. The most effective concentration - 20 nanomol - is in the range of peak values that you can find in the blood of people with healthy sleep. The figure below reveals how melatonin inhibited the growth of the T47D cells. At a concentration of 20 nanomoles, melatonin inhibited the conversion of testosterone into estradiol just as well as letrozole. So it appears that melatonin is an aromatase inhibitor, and a damn good one. The researchers also experimented with resveratrol, and that substance blocked the aromatase enzyme as well. But the concentrations that were needed for resveratrol to inhibit aromatase were too high to be interesting. Conclusion "The melatonin concentration of 20 nanomol and resveratrol concentration of 20 micromol have aromatase inhibitory effect as potent as 20 nanomol of letrozole, which is a clinically used anti-aromatase drug in breast cancer treatment." "The selective estrogen enzyme modulator mechanism of action of melatonin may offer potential advantages for cancer treatment."