Since no one else has tackled this... I would guess that practices depend on how confident the staff is that the animals would be secure at night, not only in terms of escaping or tearing up their exhibit, but also in terms of (human) intruders and predators. There would also be concerns about animals fighting with no one around to intervene, and perhaps a whole additional list of concerns. Highly situational, I would say. But it is sad to see animals out in a spacious, varied exhibit for only perhaps 8-10 hours a day and confined to a much smaller and more boring space the rest of the time.