This would be a correct conjecture. Codeine and Heroin while varying in potency (with Codeine being quite mild and Heroin being heavy), they are both a type of Opioid. Opioids are drugs derived from the active substance from Opium, which is used as a downer in both illicit and pharmaceutical drugs. Downer's are actually called depressants and are designed to depress the CNS (Central Nervous System), which does cause drowsiness if you were to take it all mildly. Two other depressants you may be aware of include Barbituates and Alcohol.