It's rarely seen at night (yes, it's because of the animals' sleeping schedules). DAK is AZA (American Zoo Association) certified, and would easily lose the certification if they did anything but private parties in Dinoland at night (which are FUN to work)
Also, I saw someone mention EMH at DAK. The only EMH that DAK does are in the morning from 8-9 on select days. They don't do EMH at night for reasons stated above. As much as I'd like to see a nighttime spectacular like World of Color at DAK, it's just impossible.