In my experience the most common 'sleeping pills', and one that is quite effective, is available OTC as an antihistamine: Benadryl/Diphenhydramine. It typically comes in 25mg tablets or capsules and 25 to 50mg will lay most adults out quite handily. It is one of the most commonly used drugs in many hospitals where it is often the drug of choice for patients who need a little something to sleep in the hospital environment.

Diphenhydramine is one of those 'must-haves' in my medical kits. As an antihistamine it often works where more modern antihistamines don't. It works reliably as a sleep aid, controls motion sickness, is an emergency stop-gap for asthma or severe allergic reaction, and even has a role as topical analgesic. It is also cheap with a 100 count bottle of generic going for all of $4.

I go for the capsules because the option of opening them up and using the powder can speed absorption and makes swallowing easier. The powder can be added to food or drink. Diphenhydramine is quite bitter so mix in just enough to ease swallowing. I've been told, don't know for sure and I'll get professional advice before I try it, you can snort the powder or mix with liquid and take as an enema. Which might be useful if the patient can't hold food or drink down.