Going out on a limb here, but I’m assuming it’s meant to be “why [exogenous] melatonin is bad for you”.

Melatonin is naturally secreted by the pineal gland, which like other hormones gets downregulated when taken exogenously over long periods of time.

There’s also studies showing a downregulation in dopamine and seratonin with long term use of melatonin.

Ah - here's the link - https://impossible.co/blogs/health/melatonin-side-effects