You can take too much melatonin in a single go, but you can also take too much in smaller doses, over a long period of time. Melatonin should be taken on "a short-term basis if you’re experiencing insomnia, want to overcome jet lag, or are a night owl who needs to get to bed earlier and wake up earlier, such as for work or school," Johns Hopkins sleep expert Luis F. Buenaver, PhD, CBSM , said in an interview on the Johns Hopkins website. Experts don't know what could happen if you take it daily over a long period of time ; until more research is done, experts are recommending staying on the safe side. If taking melatonin does appear to help your sleep problems, and your doctor has signed off on it, then it’s okay for most people to take each night for around one to two months, according to Buenaver. "After that, stop and see how your sleep is," he said. If you're still tossing and turning, head back to your doctor to discuss your options.