Why do certain medications cause nightmares or "unusual dreams" as a side effect?

Changed sleep-cycle: A variety of medications change sleep patterns, which go through various stages through the night. Disruptions in that pattern can lead to a range of problems including must less "restful" sleep as well as nightmares, sleep walking, and other atypical behaviors. Sometimes changing the dose or timing of when you take the medicataion can help. Speak with your doctor about a possible change.