Probably more than you wanted to know:
As mentioned in another thread I do not use melatonin for sleep purposes.
This a self-experiment based on John Hemmings’s experience. He takes very high doses and is still alive and book I read a long time ago:
“Melatonin: Breakthrough Discoveries That Can Help You Combat Aging, Boost Your Immune System, Reduce Your Risk of Cancer and Heart Disease, Get a Better Night’s Sleep Paperback – July 1, 1996 by Russel J. Reiter (Author), Jo Robinson (Author)”
and on a recent Kindle book:
" Extreme Dose! Melatonin The Miracle Anti-Aging Hormone Anti-Alzheimer’s Hormone Anti-Baldness Hormone Menopause Reversal Hormone Kindle Edition"
These two books tend to be mostly anecdotal and are not based on good scientific studies.
That is why I say it’s a self-experiment.
There are more recent studies indicating the benefits and safety of high-dose melatonin.
“Beyond its sleep and chronobiotic properties, melatonin is a potent antioxidant9 and has the ability to cross the blood-brain barrier,10 with suggested anti-amyloid properties. Due to this, melatonin has been increasingly investigated in many varying conditions, including cancer, cardiometabolic conditions and neurodegenerative diseases at higher doses, where there is less documentation of its safety. Doses ranging from 30 to 100 mg are being suggested or tested for effectiveness in a range of conditions and ages”
“Based on this limited evidence, melatonin appears to have a good safety profile. Better safety reporting in future long-term trials is needed to confirm this as our confidence limits were very wide due to the paucity of suitable data.”
There is some evidence of high-dose melatonin improving insulin sensitivity, etc.
A recent meta-analysis concluded:
“Conclusions: Our results showed that melatonin supplementation was useful for reducing diabetes parameters when compared to placebo.”
“Melatonin is a potent natural antioxidant and anti-inflammatory agent that protects against toxic side effects of radiation and chemotherapy”
"Safety of higher doses of melatonin in adults: A systematic review and meta- analysis