What Do Carnivores Dream About?

Anyone with a house cat already knows the simple truth of a new study on the comparative physiology of sleep across mammals. Carnivores sleep much more than would be expected compared to herbivores and omnivores:

Carnivores:

Herbivores:

Omnivores:

Of course, these data don't answer the original question, but only raise more questions.

This website uses the data to conclude that humans should eat a vegetarian diet, because human sleep requirements match those on the Herbivore sleep regression, and we don't get as much sleep as other omnivores of the same body weight.  But based on discussion in The Story of the Human Body: Evolution, Health, and Disease by Daniel E. Lieberman humans in their modern form -- i.e. that would be recognizable as human today -- cooked their food, which consisted of vegetables and meat and fish.  None of the other animals in this study cook their food.  Perhaps our higher quality diet allows us to spend less time digesting and more time alert?

But herbivores spend very little time asleep!  In fact, from this data, one might conclude that a limit on the size of herbivores is the number of hours in a day.... the largest herbivores spend almost the entire day (and much of the night) awake and, probably, eating.  In contrast, carnivores sleep the most, presumably because they can satisfy their nutritional requirements with less time and effort.

Why don't humans sleep more?