The longest cranial nerve in the body, the vagus nerve activates the body's relaxation response to help regulate stress. It stays in your system longer than other sleep medications, so it’s more likely to cause that next morning “hangover” feeling of lingering drowsiness. It stays in your system longer than other sleep medications, so it’s more likely to cause that next morning “hangover” feeling of lingering drowsiness. Diphenhydramine , used in OTC medications like Benadryl and ZzzQuil, is also one of the most common OTC sleep medications and can be used for allergies too. , used in OTC medications like Benadryl and ZzzQuil, is also one of the most common OTC sleep medications and can be used for allergies too.