When you regulary use beadryl for a large period of time, you're putting yourself at risk statistically for suffering the more severe side effects (see http://www.drugs.com/pro/diphenhydramine.html for side effects). Prolonged use also simply creates a growing tolerance to the drug over time. You'll require more benadryl to get the same reaction you've been getting. Note that prolonged use is beyond simply using it on the bad nights. It's more like taking it regularly more than twice a week.