I know it has been used as a sleep medication in some prisons. Standard sleeping pills, like benzodiazepine drugs (ex: Restoril,) are generally not allowed to be used in prisons.

Hydroxyzine is an antihistamine, related to Benadryl, which is also sometimes used as a sleeping pill. Hydroxyzine has been found to reduce anxiety.

In prison they give pretty large doses of both hydroxyzine and Benadryl. Can cause you to wake up with an uncomfortably dry mouth, which BTW is bad for your teeth.