I try not to eat very much when I'm up late...if I do I'll grab something on the healthier side; fresh fruit salad or avocados or celery a lot of the time. Occasionally white cheddar popcorn, but that's about as "junky" as I'll for midnight snacking.
I frequently suffer from insomnia, but during the last few years my sleeping patterns have greatly improved since I learnt how to relax before going to bed. I have noticed that when I can't find any rest at night, I quickly fall asleep after eating a couple of tablespoons of something relatively heavy, such as cooked quinoa or pumpkin. Even a quarter of a bowl with noodles will do the trick. For me it's the comforting feeling of the warmth inside my stomach that puts me to sleep. But the idea is to only have a small quantity, so as not to suffer from any nightmares or gas during the night.