There is no real way to quantify strength among over-the-counter sleep aids; however, the most popular sleep aids include those that contain diphenhydramine, which is an antihistamine that is useful in promoting sleep due to its sedating properties. Mayo Clinic states that a number of OTC sleep aids contain diphenhydramine, including Unisom Sleep Gels, Tylenol PM and others.
Doxylamine succinate is another type of sedative/antihistamine that is used in sleep aids. Melatonin is a type of hormone that is sometimes used for sleep remedies by helping users to control sleep and wake cycles. Valerian supplements are a natural type of sleep aid that are derived from the Valerian plant, according to Mayo Clinic.