Does taking opiate increase the likelihood of getting nightmares?

Not necessarily: Taking a lot of opiates at night, or even smaller doses can cause insomnia. The person is in a trance like state and may have excess energy and feel like doing stuff.

It could: Hard to generalize accross all opiates. When Meperidine (demerol) is metabolized in the body it creates normeperidine which can produce nightmares, anxiety, and other symptoms. These symptoms are most likely to occur after a few days of treatment with meperidine.