Don’t eat within a few hours of going to bed. It’s likely you have light acid reflux and sleeping with a full stomach, particularly when it’s full of something so rich and acidic, will compound the problem significantly.

Also I’d recommend lowering the amount you’re eating. Eat just until you’re no longer hungry, not to the point that you’re full. If you want an extra slice later as a snack cold pizza is awesome, but you can’t un-eat the slice or slices that are punishing your guts.