Tolerance to the sleep-inducing effects of Benadryl builds up fairly quickly. In this double-blind study, people given 50 mg of diphenhydramine (the active ingredient in Benadryl) got really sleepy the first few times, but after doing this for four days in a row, the effects were indistinguishable from a placebo. Benadryl usually comes in 25 mg tablets, so that's two pills per night.