I am really unclear why everyone is taking a medication that can do a host of things, but yet its primary use is not to be a sleep-aid. If you need assistance to sleep take a sleep-aid. Neither Dramamine nor Benadryl are indicated for the use of sleep assistance. Why would you take a drug in hopes of getting a side effect (sleeping) and ingesting chemicals that you have no need for and could also have other unintended effects.
Benedryl is a name-brand for a drug that is indicated for two uses*- for its antihistamine effect and its ability to induce drowsiness. Hence its marketing in products such as Tylenol PM. In the US, diphenhydramine is approved by the FDA to be marketed as a sleep aid. Just like the medication blend in Excedrin is now also marketed under the name "Execdrin Migraine" - it's the same thing, but the FDA determined that it also is effective against migraines in addition to garden-variety headaches and other pain. How is it that you don't consider its use as a sleep aid to be appropriate?
There is no difference in taking Benedryl or any diphenhydramine OTC medication and taking Tylenol PM - EXCEPT that if you are just interested in the "PM" part, you aren't also ingesting acetaminophen. If anything, taking diphenhydramine in its allergy preparation (without other added meds like pain relievers or decongestants) is BETTER. Not only that, it's often cheaper to buy the generic version of benedryl than it is to buy the generic version of the PM version of pain relievers. And they come in smaller pills that are easier to swallow.
*The third use is to relieve motion sickness, but since the preparation is slightly different, I didn't include it. Any information related to diphenhydramine indications will also include motion sickness, though.
To the OP - I don't know what your seating arrangements are for your overnight flight. However, what I will tell you is that I often find lying down (either reclining my seat or lying flat, as I'm often doing on overnight flights since those are almost always business or first class) increases my motion sickness. So if you are at all prone to motion sickness, you may want to bring a med along to relieve it.
In general, I find that non-binding clothing and warm socks help me feel more comfortable. I don't drink much alcohol anyway, but I avoid it on flights because it makes me feel very dehydrated, which drains my energy.