Reviewers had three trials, and would not call it effective or safe
A review of randomised placebo-controlled trials published between 2003 and 2014 found eighteen. Three tested diphenhydramine, and its efficacy and safety were judged not well supported.
What the study found
Eleven years of sleep-aid trials produced three on diphenhydramine
A review searched for randomised placebo-controlled trials of over-the-counter sleep aids published between 2003 and 2014 and found eighteen. Three tested diphenhydramine, and the authors concluded its efficacy and safety are not well supported. Slow-release melatonin came out ahead.
The caveat · One database, searched from 2003
The search deliberately began at 2003, so three is a count inside that window rather than of the whole literature. One database was searched. The second author worked for a company selling over-the-counter sleep products, melatonin among them, though its own diphenhydramine line comes off badly too.