Melatonin is made by pretty much every complex lifeform on Earth, including plants. It's a very ancient molecule.

In addition to being involved in circadian rhythm, melatonin has powerful (positive) effects on health. Low melatonin levels are known to increase the risk of cancer, and in recent years high-dose -- often 100 mg or more -- melatonin has been successfully used in cancer therapy.

From https://riordanclinic.org/wp-content/uploads/2019/03/3-3-Shallenberger-High-Dose-Melatonin-Therapy-An-Ideal-Adjuvant.pdf --

Melatonin Facts

 Produced and released in pineal gland in darkness. Immediately suppressed by all light but red light.

 Not a soporific. You can take it during the day, as well as at bedtime, without any adverse effect.

 Side effects: no serious side effects. Some patients report sleep disturbances and AM sleepiness.

 Dr. Pierpaoli, one of the world’s leading melatonin researchers, has successfully used daily dosages ranging from 0.1 to 200 mg. That’s a 2,000-fold difference between the lowest dose and the highest! Studies on mice show that even at astronomical doses of 300 mg per day for two years, there were no side effects.