I initially declared the nightmare-via-pizza theory to be true, but then later considered the placebo effect. Perhaps in this scenario it would be considered the nicebo effect. Nonetheless, the experiments were successful in producing the hypothesized outcome. Whether that outcome was actually caused by the delicious bedtime snack or my mind anticipating nightmares in leu of eating pizza, you can still reason it out to pizza'a fault.
a) the consumption and digestion of pizza directly caused the induction of nightmares
b) the knowledge of my consumption of pizza influenced my subconscious to induce nightmares because I thought pizza would cause them
Either way, the pizza still caused the nightmares. This is true because if I hadn't eaten the pizza, I would not have had any nightmares, scenario a) or b).
For the record, I very rarely have dreams that can be considered nightmares: one every one to two years. Having three in one week does not follow this trend.
Science isn't perfect and there is still more testing to be done. But, I will make the statement that sounds as though it's straight off a prescription drug commercial: Consumption of pizza before bed has been found to increase the risk and likelihood of experiencing nightmares. If you have a heart condition, please ask your doctor before starting pizza.