Antihistamines are a bad move for sleep now.
The first-generation, sedating antihistamines are all cholinergics. The data are really clear that use of cholinergics contributes to dementia risk in a cumulative manner. The risk increases for everyone, and if dementia was already in the cards, the onset can be sped up and the severity can be increased. A Benadryl during an allergy attack is one thing, but every night is a different beast.
The common ones are diphenhydramine (Benadryl), doxylamine (Unisom), dimenhydrinate (Dramamine), hydroxyzine, chlorpheniramine, promethazine, and so on.
They also really fuck with sleep architecture, so it may end up making the problem worse.