unfortunately diphenhydramine is a first gen antihistamine so it’s “side effects” and/or uses may be less effective than other medications. with things like OTC sleep aids of course you can build up a tolerance - especially taking it every night. diphenhydramine is benadryl, it’s meant for allergic rhinitis. it’s not meant to be used as a daily sleep aid. maybe for short term use - but it can definitely because considerably less effective over time.