Isn’t this cutie absolutely adorable? It is very uncommon for cats to sleep while standing up. The animals which sleep standing up are all grazers: horses, giraffes, yaks; and the reason they sleep standing up has a lot to do with simple anatomy. Grazing quadrupeds do not have a full diaphragm like we do, that separates their guts entirely from their lungs. Also, in many animals the lungs are attached directly to the ribs and breathing must involve motion of the ribs. Now, when the animal lies down, this compresses everything: the ribs get squished in along where they lie, and the guts get squished up inside them, and the guts tend to squish up around the other organs-including the lungs, and also put further pressure on the heart. An adult elephant will actually die if it lies down for more than a few hours. The smaller the animal is, the less this is a problem.