Are there any short acting anti-histamines? (i often have insomnia in the early morning when it is too late to take a unisom.)
Yes. Sounds like you are looking for an antihistamine for its sedative effects. Diphenhydramine is marketed as Benadryl (diphenhydramine) for treatment of hives and allergic rhinitis and sominex for insomnia. The dose is the same 25-50 mg for adults. Diphenhydramine begins to work in 30 minutes. Its effects last about 4-6 hours.
What kind of Unisom? From: wiki: doxylamine is a member of the ethanolamine class of antihistamines, it is the most effective over-the-counter sedative available in the United States and is more sedating than some prescription hypnotics. Doxylamine succinate is the active ingredient in the over-the-counter sleep-aid tablets branded as unisom; however, the gel-cap form contains Diphenhydramine hydrochloride instead.