I'd recommend against regularly taking anticholinergic drugs like diphenhydramine because they've been fairly clearly linked to cognitive decline in the elderly when taken regularly: https://en.wikipedia.org/wiki/Diphenhydramine#Adverse_effect...

It's worth noting that many allergy meds (like diphenhydramine) are anticholinergic, but only ones which pass the blood-brain barrier are linked to cognitive decline. So newer allergy meds like cetirizine and loratadine are better.

Your link says anticholinergic use later in life has been linked to cognitive decline, not that use by young people results in cognitive decline later in life.

I linked to Wikipedia, but I originally read the abstract of a different study, which might have examined diphenhydramine specifically rather than many anticholinergic drugs. The abstract of the article cited on Wikipedia makes it clear that it's a cumulative effect, and that they studied outcomes in older adults. So I think Wikipedia describes the research incorrectly.

https://pubmed.ncbi.nlm.nih.gov/25879993/

For those people who feel groggy or nauseous/headachy in the morning after Melatonin, try taking 0.3mg. You might find it has the same effect with less pronounced side effects.

(I am not a doctor)

I have tested different doses quite a bit. 5mg makes me feel groggy the next day. <0.5mg seems to do morning. ~1mg seems like the sweet spot for me.