Unisom (doxylamine, or sometimes diphenhydramine) is an antihistamine. The effects of antihistamines are nothing like those of benzodiazepines, so you are barking up the wrong tree if that is what you are looking for. Most likely, you will experience mild sedation along with some other annoying PNS effects that go along with antihistamine use.
With that said, if you have insomnia, taking 100mg of doxylamine or diphenhydramine should be good enough to help you get to sleep. Keep in mind, that higher doses of antihistamines can cause delirium, hallucinations etc. and from what I heard, the delirious states induced by high dose antihistamine use is NOT FUN or RECREATIONAL