I too have taken Melatonin for just about 20 years.
I read a book on it many years ago with all the studies and found nothing bad about it. I sleep well with it. I take 3 mg every night. ( 7 hour sleep and refreshed in the morning).
Regarding Deep Sleep Deprivation:
I always had a sneaking suspicion I was not getting enough deep sleep at night, but it wasn't until I got a smartwatch recently that I saw how shockingly little I get in the deep sleep cycle each night. While the recommended range for adults in my age group is 1.75 to 2.25 hours of deep sleep each night, I was averaging 13 minutes up to 28 minutes the past 6 nights.
I started my smartwatch sleep monitoring without any usual Magnesium or Melatonin for the first couple of nights and my deep sleep average was a dismal 13 to 17 minutes. Once I added magnesium, it went up about 10 minutes. Last night I took 40 mg of melatonin (my usual dose when I take it) and my deep sleep total amount went up to 47 minutes last night. Woo hoo! Still under but better. I'll report back as I experiment more with supplements, herbs and exercise. I highly recommend a smartwatch if you're interested in how much deep, core, and REM sleep you're getting each night. Very interesting.
And a big thanks to Art for all of his articles on melatonin. I never would have experimented with higher doses, had it not been for all of his research and postings!