There's no exact dosage of melatonin that everyone should take, as it can vary based on factors such as gender, age, health conditions, body size and more. According to the NIH, no effective dosing has been established, and dosing in studies has ranged from 0.1 up to 10 milligrams.  The National Sleep Foundation recommends a dose of 0.2 milligrams to 5 milligrams for adults, although it's not clear where that determination came from. If you plan to take melatonin, try starting with the smallest possible dose and working your way up to a dose that helps you fall asleep but doesn't cause any side effects.  Keep in mind that the FDA doesn't regulate melatonin, so what you see on the product label may not be what you get.