Anti-histamins knock me out. I was prescribed them for morning sickness when expecting my second child. I took three doses as prescribed - then threw them away, when i finally woke up. A vomiting mother was saferlooking after a toddler rather than a comatose one. Fortuately I took the 3 recommended doses over a weekend when DH was home and he took over all child cars saying wasn't safe to be left with a toddler, or baby for that matter.
I travel to France a lot by ferry and, as I am prone to seasickness. I always have seasick tablets with me, these are primarily anti-histamins. I only take them when the rough sea has started because they knock me out for the rest of the voyage and often the rest of the day/night.
We all react differently to medications. One person's knockout drops are somebody elses party poppers.