Final answer:
A drug that causes decreased heart rate, respiratory depression, and sleepiness is a depressant. Depressants calm the brain and are commonly used to treat anxiety and insomnia, but can have serious side effects at high doses.
Step-by-step explanation:
The drug that would result in a decreased heart rate, depressed respiration, and sleepiness is a type of depressant. Depressants are a class of psychoactive drugs that suppress central nervous system activity, leading to effects such as reduced anxiety, sedation, and sleepiness. Common examples of depressants include barbiturates, benzodiazepines, and opioids. Such drugs work by serving as agonists of the gamma-Aminobutyric acid (GABA) neurotransmitter system, which has a calming effect on the brain. While beneficial for treating anxiety and promoting sleep, depressants can also result in undesirable effects like ataxia, cognitive/memory impairment, and in high doses, decreased respiratory function and even death.