Dr. Crawford is studying the side effects of a drug on the stages of sleep. He finds that the drug inhibits REM sleep but has no effect on nREM sleep. What is the drug's most likely mechanism of action?
1) The drug is an acetylcholine antagonist.
2) The drug is a serotonin antagonist.
3) The drug is a norepinephrine antagonist.
4) The drug is a dopamine antagonist.