REM sleep behavior disorder is not a mental illness. Rather, it is a parasomnia, which is an abnormal physical activity that occurs during sleep or as someone is falling asleep or waking up. That said, RBD can occur alongside psychiatric problems, and there have been cases in which specialists mistook RBD for a psychiatric condition. Additionally, it is fairly common for RBD to appear in people with depression who take certain types of antidepressant drugs. But in these cases, it is most often the drug and not the depression itself that causes the SBD.