“Many OTC sleep aids—such as Benadryl and Tylenol PM—contain diphenhydramine,” says Dr. Donovan Maust, co-author of the recent study and an assistant professor of psychiatry at Michigan Medicine. Diphenhydramine is an anticholinergic drug, which means it blocks activity of a brain chemical called acetylcholine, which plays a role in muscle activation and also in brain functions like alertness, learning and memory, Maust says.

More about Maust quoted in Time for sleeping pill use among U.S. older adults >