I quit using Chantix. I'm quit now for over 300 days. I never had insomnia with Chantix... and did have nausea if I took it on an empty stomach. Once I learned to take it with a meal, no more problems. I had menopause related insomnia for years... so I was worried about having insomnia again, but never did. In fact, I slept better than ever on Chantix.
I know a lot of people who've taken Chantix to quit and have been successful. None of us had nightmares... but we did have beautiful and/or interesting colorful dreams. You may have something else going on... and you may just be an exhausted new mom. Twins!!!!!!!!! Now, there's a good reason not to get a decent night's sleep. Being overtired is a good reason not to be sleeping.
Not sure if you drink caffinated soda or coffees or teas, but I suggest cutting way back on therm. I think you need to check with your doctor, hon. Good luck and keep up the great work.