This is a general opening up of my OP and the (not many serious) posts in Do tapeworms sleep? If not, why not?, which for some reason was a burning question at the time.
Query 1. Which animal spends the most time asleep in the animal kingdom? Ditto, for least.
The definition even to get to an answer must have certain parameters I guess, both physiologically–e.g., I don’t think “hibernation” counts, either for bears or everybody’s favorite, the tardigrade [no offense to bear lovers])–and, as in query 2:
Query 2: Ethology is critical in coming up with answers to Query 1, I think. Woody Allen’s biblical restatement exemplifies it: “the lion will lie down with the lamb, but the lamb won’t get much sleep.”
One problem you’re going to have is that “sleep” is not the same for all species, and various animals have different levels and types of semi-conscious rest.
There are some animals that never do anything that can be described as sleeping, and some that can never really be described as awake, either. Does a sponge sleep never, or always?
Horses are a prey animal I know, and they do sleep less than their predators.
Horses only sleep 2-3 hours out of a 24-hour day. And they do that standing up. They actually lie down to sleep only once a week or so, because the laying down & getting up is physically taxing, and puts them in a more vulnerable position (less defensible, and slow to get up). This sleep might come at any time day or night – horses are semi-nocturnal (thus the big eyes) in the wild. (Domesticated horses tend to sleep more at night, because they are often kept inside stalls in a barn at night.)
But horses do catnap or doze off frequently during the day. They will be grazing out in the pasture, and will doze off for a short nap just standing there. But horses stay in herd groups, and not all nap at the same time – at least a few of the horses in the herd are wide awake at any time. And horses are capable of waking and immediately taking off at a dead run.
I wonder if the size of a prey animal plays a role in how much it sleeps. Looking at the list provided by Riemann, large prey animal like giraffe, cows, horses, etc sleep very little but smaller prey animal (rabbits, mice, chipmunks, etc) sleep far more. I wonder if that is a pattern seen in the wild and if so, why it happens. I’d guess part of it is that smaller animals are able to burrow and hide from predators. A mouse can hide from a cat, a horse can’t hide from a coyote.
Edit: I should’ve checked the link. It answers the question.
Does a low probability of encountering a predator matter that much, though? We’re considering a daily activity, with fatal consequences, so even rare events will happen eventually. A horse’s predators would presumably be wolves, big cats? They can go anywhere a horse can go, so the horse’s only tactic is constant vigilance and running away.
My 15-year-old cat sleeps about 15 hours a day. She sleeps more as she ages. I figured as a hunting animal, the cat’s instincts tell her to conserve as much energy as possible when not hunting. Stalking, pouncing, killing, and devouring require more bursts of intense energy than standing around grazing. She as an indoor/outdoor cat, but I think she’s figured out by now she’s gonna get a nice meal whether she hunts or not!
