My dog is almost 3 years old and she has always been rambunctious and high energy. I know that she is reaching adulthood soon so she has gotten a bit more mellow, but lately she has been sleeping/laying around for +/- 18 hours a day. She is a larger mixed breed, but should I be concerned she has slowed down too much? She has been eating/drinking well and when I work at it a little she plays well for a little bit, but then just goes straight for the door inside and to her bed. Sometimes she doesn't sleep but just lays there and looks sad, and sometimes she whines. Before I resort to taking her to the vet, is it possible dogs can have arbitrary depression? Pic related