Expense isn't an angle I've heard Nocturnal houses attacked before. From what I gather, the decline of Nocturnal houses is more so due to the fact they aren't the most popular exhibits, are oftentimes difficult for crowd control (i.e. people can't always see where they are going easily), and a general decline in small nocturnal species being present in collections. There is a shift, however, towards more zoos having small stand-alone Nocturnal exhibits with a few species in them as parts of other complexes, so the concept of Nocturnal exhibits isn't going away completely, just shifting towards more smaller-scale exhibits for Nocturnal Prosimians, Kiwis, Bats, etc. as parts of larger complexes. A few other issues I have with Nocturnal houses are that visibility tends to be an issue (I've found its on average harder to find animals in nocturnal exhibits that any other style of exhibitry), they're terrible for photography (especially if you don't have fancy lenses, etc.), and most species, especially in captivity, don't follow strict light-dark cycles for when they're awake/active. Nocturnal houses would be great if they yielded higher levels of activity in the species, but oftentimes I've found that in a Nocturnal exhibit visitors are looking at a sleeping animal in the dark instead of a sleeping animal in the light. Back when I volunteered at a zoo that had a Nocturnal exhibit, the line I used was always that "the animals sleep when they are tired, and are awake when they aren't", as so many of the animal's biological needs are taken care of in captivity that oftentimes the animals will deviate more from their "natural" sleep cycle, oftentimes determined due to temperature, food availability, and prevalence of other threats.I have heard before that nocturnal houses are expensive to maintain. Does anybody know why this is compared to other indoor exhibits? All I can think of at the moment is that leaving the lights on a night must cost a large sum of electricity.