If diphenhydramine is a sleeping pill and an antihistamine, then why is it safe to take daily for allergy but not when insomnia is the issue?

Good question.: As long as you don't exceed the dose or frequency on the label - they are equally risky/safe. Perhaps the concern more with insomnia than allergy is that sleep is a combo of physical and behavioral causes while allergy is considered all physical. So, some may think you should 1st try good sleep habits/hygiene before pills. Also they may fear dependence more w/ insomnia. Sharp thinking!