The sedation had worn off by day four, in fifteen healthy men
On 50 mg of diphenhydramine twice daily, fifteen men were measurably sleepier than on placebo on day one and indistinguishable from it by day four.
What the study found
The sedation wears off completely by day four
That it stops working is the ordinary pharmacology, not a personal failing. Fifteen healthy men on 50 mg of diphenhydramine twice daily were measurably sleepier than on placebo on day one and indistinguishable by day four. The geriatrics guideline names that as a reason to avoid it.
The caveat · Daytime sleepiness, not bedtime use
Fifteen healthy men, no women, nobody with a sleep problem. The dosing was twice daily and the outcome was daytime sleepiness, so a nightly bedtime habit is an inference rather than something this trial watched. Four days shows tolerance arriving and says nothing about weeks.