The invention provides a method for diagnosing a disorder or disease associated with an imbalance of monoamines in a subject, or of diagnosing susceptibility of the subject to the same, wherein the method comprising (a) providing a sample from a subject to be tested, (b) measuring the levels of dopamine, melatonin and/or serotonin in the sample; and (c) comparing the levels measured in step (b) with levels of dopamine, melatonin and/or serotonin associated with a reference population of subjects, wherein a difference in one or more of the levels measured in step (b) with the levels of dopamine, melatonin and serotonin associated with the reference population of subjects is indicative of a positive diagnosis. The invention further provides a method of prevention or treatment of a disorder or disease associated with an imbalance of monoamines, and kits for the same.