Ever tried doxylamine? It's also a first generation antihistamine, although the standard OTC dose will supposedly cause drowsiness for almost 8 hours. It's the ingredient in Unisom SleepTabs, but when I use it I typically get the Costco house brand.
It's also a pretty effective antihistamine, although I find none sold for allergies or cold symptoms. My wife had tried it a few times and it knocks her out cold for maybe 12 hours. However, she's also convinced that diphenhydramine sold as a sleep aid is more effective for that than if it's sold as an allergy medication.
The maker of Unisom says that the version with diphenhydramine shouldn't be used in place of an allergy medication, but it's actually pretty effective for that if you know what dosage you need and how long between doses. I literally buy 600 25 mg tablets at Costco for about $3.50. Any doctor will say it's fine to use this as a sleep aid in place of a product labeled as a sleep aid. However, you need to know what the proper dosage is, although it's not that hard to figure out.