Regarding over-the-counter products, the active ingredient of over-the-counter sleep aids like Tylenol PM, Benadryl and Unisom is diphenhydramine, an antihistamine. It is generally safe and non-addicting. There are certain people who should be cautious with using it, such as the elderly, those with asthma and those with enlarged prostate, as these people may have significant side effects. Ask your doctor if diphenhydramine is safe for you.
Tylenol PM, specifically, has acetaminophen (the active ingredient in Tylenol) combined with diphenhydramine. Acetaminophen, when taken in large doses, can potentially affect the liver so should be used judiciously. Benadryl, Unisom and Sominex, for example, do not contain acetaminophen, only diphenhydramine. As with any sleep aid, diphenhydramine should not be mixed with other sedatives or alcohol.
Keep in mind that requiring a sleep aid to fall asleep is not the same as being chemically dependent on, or addicted to, a medication. If you do require a sleep aid on a long-term basis, you should talk to your doctor about the best way to manage your insomnia.