Melatonin. Molecular model of the hormone melatonin. Atoms are represented as spheres and are colour-coded: carbon (light blue), hydrogen (yellow), oxygen (red) and nitrogen (dark blue). Melatonin is secreted by the pineal gland in the brain that controls the body's biological rhythm. It is secreted at night and induces sleep. Pills containing melatonin can be taken to prevent jetlag. In middle age, melatonin secretion drops off and may be responsible for aging symptoms such as insomnia and irritability.