Melatonin has two main types of receptor called ML1 and ML2 of high and low affinity respectively. Receptors have been found in a wide range of tissues including the brain (e.g. the suprachiasmatic nucleus and the hypothalamus), the pituitary gland, blood vessels, the adrenal glands, ovaries and testes. In addition it is thought to act on specific intracellular molecules (such as calmodulin, a protein in cells that alters intracellular calcium levels).
The precise functions of melatonin have not been identified but some actions include: