A nocturnal house is a building in a zoo or research establishment where nocturnal animals are kept. The unique feature of buildings of this type is that the lighting within is isolated from the outside and reversed; i.e. it is dark during the day and lit at night. This is to enable visitors and researchers to more conveniently study nocturnal animals during daylight hours.
Internally, a building usually consists of several glass-walled enclosures containing a replica of the animals' normal environments. In the case of burrowing animals, often their tunnels are 'half-glassed' so the animals can be observed while underground.