Hello, Allergy is an answer of your organism towards an antigen (something different from what your body recognizes as its own) in an excessive or exaggerated way. Any component or substance that comes in contact with the organism can then be called antigen, according to this concept. Antihistamines, although their main action is to counteract the effects of an allergy, some also produce sedation. To answer your question, I must first correct you. The main effect of diphenhydramine is to alleviate allergies, that is, antihistaminic. Its side effect or rather, the adverse effect would be sedation. So, the truth is that sleep aid is not a good business name because it creates confusion. So if you use that drug in order to reconcile the dream you should not, since there are medications for that type of problem more specific. Sleep Aid, as you have said is a combination of two drugs (acetaminophen, diphenhydramine) for the symptomatic relief of many causes, usually viral infectious diseases. According to what I said before, if we recognize these viruses as an antigen (actually a fact); In addition to a headache, it can also trigger allergic reactions (rhinorrhea, eye redness and itching). Why is the drug than in a sleeping tablet? The answer to that, is: that tablet (Sleep Aid) is not used for that purpose (sleep) although it can help you sleep better by relieving the symptoms. To answer your other question, You can also take it for your headache, but remember that it produces that side effect of sedation. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Luis Alberto Urdaneta Arenas, General & Family Physician