Be careful if you take the antihistamine and plan to drink alcoholic beverages. The combination could make you very sleepy or conversely you could have a paradoxical affect and it might make you agitated.
Ruth

thanks.
to be honest, i'll never have more than one drink or two max when flying as i get super-dehydrated. even without alcohol i tend to drink a LOT of water on planes.

Bear in mind that over-the-counter cold medication in Japan is typically half as powerful as the over-the-counter drug with the same name in the U.S., so you have to take twice as much.

Ruth has very good advice here.
Though just a word of warning from my experience: it just happens that I am one of those people who don't get knocked from Ambien, and I ended up with amnesia... it is rather scary because I found myself sitting in a convoluted way, it makes you wonder what you may have done in the 4 hours or so that you have just lost!!
Xanax is indeed short-acting. Clonazepam (Klonopin) has the exact same effects (both are benzodiazepines, and so is Valium btw) but it is longer-lasting and has more of a hypnotic (sleepy) effect. Valium (Diazepam) also works but it is very long-lasting.
Neither anti-histamines nor melatonin have done anything for my insomnia. Anti-histamines can also make many people rather stupid...
In any case, alcohol is a big no-no with any of those.