One more relevant study, but on the health effects of long term melatonin use:

https://newsroom.heart.org/news/long-term-use-of-melatonin-s...

"The main analysis found:

* Among adults with insomnia, those whose electronic health records indicated long-term melatonin use (12 months or more) had about a 90% higher chance of incident heart failure over 5 years compared with matched non-users (4.6% vs. 2.7%, respectively). * There was a similar result (82% higher) when researchers analyzed people who had at least 2 melatonin prescriptions filled at least 90 days apart. (Melatonin is only available by prescription in the United Kingdom.)

A secondary analysis found:

* Participants taking melatonin were nearly 3.5 times as likely to be hospitalized for heart failure when compared to those not taking melatonin (19.0% vs. 6.6%, respectively). * Participants in the melatonin group were nearly twice as likely to die from any cause than those in the non-melatonin group (7.8% vs. 4.3%, respectively) over the 5-year period."

However they were not able to control for severity of the insomnia and used dosage, because that data weren't in the dataset.