Those kind readers who followed my recent sermon are familiar with the grumpy barncat in its concluding Normaphoto.
Theta Waves indicate the state of very deep relaxation of hypnosis and Dream Sleep. Brain waves are slowed to a frequency of 4-7 Hertz (cycles per second). Cat brains are about 1/40th the size of human brains so they might be easier to relax. Conversely, this theory accords each human skull an equivalent of 40 cat-brains, and all they do is fight in there so we'll dismiss that notion and proceed to a graph.