Melatonin is sold in 10mg dosage, so acutely, I don't see this being a problem. That said, I wouldn't rely on a substance to help fall asleep UNLESS you have already covered the other bases, i.e. No caffeine 6-8 hours before bed, not staring at a bright phone/TV screen while falling asleep, not exercising within 1-2 hours before bed, not drinking alcohol within 1 hour before bed (if its a habitual thing), and of course other stress related things that can keep you up like not being prepared for the next day (gas, food, clothes, work, time, etc.)