This is the cat benign equivalent of sun stroke. They wander into the edge of a sun beam, their brains are shut down, and they collapse. Over several minutes you can observe their bodies reducing height and increasing breadth, maximizing the square inches exposed to the barrage of photons, and deepening the soporific effect. Only when the sun moves to another position, and sufficient cat surface is in shadow, do they revive, but they're usually dragged back into the sun and the process is repeated.
Notice in this aerial view the circles of light around the square window spots. (Click images for closer view.) This is evidence that the sun is round, and appears in our sky as a disk, not a point. If it were a triangle, this refraction distortion would be a triangle. It's caused by the fact that not all photos are coming past the edge of the window from a single angle - some come from every side of the sun circle and therefore get around the sharp edge of the square, casting this circular spot. If you've noticed the light spots on grass or sidewalk through tree leaves, they also form circles for the same reason, regardless of the odd shapes of the space they shone through between the leaves.
Working today - just took a break to note these interesting scientific phenomena