"Prayer Plants" all do this; it's called nyctinasty. Usually it's plants in the Maranta or Calathea family. There are several theories why they do it, such as

"Prayer Plants" all do this; it's called nyctinasty. Usually it's plants in the Maranta or Calathea family. There are several theories why they do it, such as water retention or discouraging predation, but they don't really know for sure.