How much melatonin is safe to take at once. 130 pounds?

Start with 1 mg. I do not recommend high doses that is often by health professionals. Melatonin is a natural chemical secreted by cells in certain parts of the brain. A poor sleep pattern, perhaps caused by change in normal body's diurnal rhythm (long air flights, night shift work, long hours of study at night) can be helped by melatonin, but suggest 1mg, 2mg, 3mg, each for 1 week; no help? Fogetaboitit-:).
Melatonin. You should stick with 0.5-5mg 1 hour before bedtime. Larger doses of melatonin can even be counterproductive and dangerous. There was a study that showed 0.5 mg of melatonin was effective while 20 mg was not. Because of the potential for side effects and interactions with medications, the safest thing to do is for you to take it under the supervision of a knowledgeable physician.