melatonin is really good, most doses start from 1-3 mg, but I have seen as high as 90 mg (yup). Current literature suggests it helps protect against the big A (Alzheimer's) by helping the regulation of the Circadian rhythm cycle. If you are waking up like a bolt, due do the epinephrine rush, (dump from the adrenals) your are in adrenal stress, and need to take steps to adjust your cortisol. Adding 5-HTP will also help a great deal. Depending on how far into health stuff your are, you could do a gut cleansing with Monolaurin so that natural serotonin your stomach helps restore the body cycle as a whole
While Benadryl and other over the counter stuff works, they are really no better than a shot of scotch, and both are CNS depressants, and liver enzyme inducers as well, and you don't even get to enjoy it. Over dosing by Melatonin from a toxic level doesn't happen, but can make you groggy the next day.
the increase or decrease of hormones is called feedback inhibition, and is much more complicated than a simple receptor shut down.