Answer: They're by and large antihistamines originally marketed for their H 1 histamine-blocking properties and found to cause drowsiness (not extraordinarily well understood, but it has to do with the reticular activating system).Traditional Unisom is doxylamine. Doxylamine, with pyridoxine (vitamin B6) added is used to treat morning sickness. Extra-strength Unisom, you might think, might be a larger dose, but it's actually a different antihistamine, diphenhydramine, which is better known under another of its brand names, Benadryl, with which I'm sure you have some familiarity.