Thursday, December 5, 2013

Not all species need it (8 hrs of sleep, that is)

So I mentioned in a recent post that this past month's Bangkok Marathon started at 2 a.m. (yes, it's true), making a good night's sleep impossible for the participants (although they probably had a great night's sleep after the race!).

The physical and cognitive benefits of sleeping allow us to work and play harder and more effectively. Sleep scientists have developed several ideas on why and how sleeping for long periods became such a key part of daily human life.

But why did humans and some animals evolve to sleep for long periods, while other species either rest periodically throughout the day or night but never conk out completely?

In other words, if sleeping is so good for us and for some other animals, including mice, why don't all animals sleep for 7-8 hours at a time each night like we do? (yes, I know, we don't all get 8 hours either...but read on!)

Sleep times vary tremendously. Some animals, such as bats, sleep up to 20 hours per day, while others, such as elephants, sleep just 3 or 4.

Compare these average sleep periods for various mammals (in captivity**) over 24 hours (courtesy of Wikipedia):

Horses – 2.9 hours
Elephants – 3+ hours
Cows – 4.0 hours
Giraffes – 4.5 hours
Seals – 6.0 hours
Humans – 8.0 hours
Rabbits – 8.4 hours
Chimpanzees – 9.7 hours
Red foxes – 9.8 hours
Dogs – 10.1 hours

Cats – 12.5 hours
House mice – 12.5 hours
Rats – 13.0 hours
Lions – 13.5 hours
Hamsters – 14.0 hours
Platypuses – 14.0 hours
Chipmunks – 15.0 hours
Giant armadillos – 18.1 hours
Little brown bats – 19.9 hours
Koalas – 20-22 hours
koalas are sleeping champions,
sleeping up to 20 hrs per day
**animals in the wild may sleep more or less than in captivity (wild sloths eat mainly leaves and sleep 9.5 hours/day, while in captivity, they sleep 16, prompting researchers to wonder whether their diet or environment (presence of predators, competitors, tree species, etc) affects the duration of their sleep.

In mammals, much of the difference in sleep duration depends on an animal's feeding style. Carnivores sleep more than herbivores, and omnivores, including humans, tend to fall somewhere in the middle.

Carnivores of a variety of body sizes sleep roughly 13 hours per day, though they range from about 9 to nearly 20 hours. If you know the term "cat nap", you won't be surprised that most wild cats (including tigers, jaguars, leopards) also catch their sleep in short intervals.

crouching tiger (ok, catnapping tiger) at
India's Kanha National Park
sleeping dragon (ok, crouching tiger) unhappy
his catnap has been disturbed

Unlike large predators like tigers that must race around to catch their food, horses, giraffes, elephants, and other larger herbivores, sleep roughly 3 – 4 hours each day, which they also get in short (e.g. 15-minute) intervals, often standing on all four legs.

Like other prey animals, they evolved to always be wary for predators, so they sleep standing up and rarely for long periods.

life's good with no predators
photo: GreenA-AnimalWorld at tumblr
Horses, at least, do need to lie down for a couple of hours every few days to get enough REM sleep, which they tend to do during the day, since their natural predators, mainly mountain lions (pumas) and wolves, were primarily nocturnal.

Overall, herbivores sleep anywhere from 3 - 13 hours each day, with much of this difference due to body size. Larger plant-eaters sleep less than small ones, and those feeding on leaves with low nutrient value, such as koalas or sloths, may sleep more to reduce their energy expenditure.

So why the differences?

  • bigger animals tend to have relatively slower metabolisms so may need less sleep
  • Click the icon for a series of 4 graphs or
    click here to read Jerome Siegel's research
    on mammal sleeping in the journal Nature
  • really large-bodied herbivores, such as elephants, have to spend many hours feeding just to take in enough calories to maintain themselves
  • large carnivores (lions, tigers) that sprint to catch their dinner and don't have to worry about getting eaten can sleep longer and more deeply than their herbivore prey
  • smaller mammals that sleep hidden in a protected burrow (mice, armadillos) or cave (bats) may be able to snooze for longer periods than those, such as zebras, that sleep out in the open
  • cows and other ruminants that sit or stand quietly for hours chewing and digesting their fibrous lunch or dinner may need less sleep.

What about cold-blooded animals?

Most sleep research has been on mammals and birds, mainly because they are most like us and their sleep state is easiest to monitor. Scientists haven't yet recorded REM (rapid eye movement) sleep in reptiles, and reptile brain patterns during non-REM sleep differ from those of mammals, so their requirements are less well understood.

Nevertheless, reptile, such as turtles, lizards, or this chameleon, respond as we do to sleep deprivation by sleeping for a longer time and more deeply when finally allowed!

sleeping chameleon in Madagascar
shows calm colors
same chameleon, bright-eyed and bushy-tailed
in the daylight

Invertebrates, such as insects and crustaceans, are also thought to sleep, though studying them seems to be even more challenging. Those cruel scientists that use sleep deprivation on rats and turtles also mistreat arthropods. Their studies have shown that, like us and other vertebrates, insects' ability to process information and respond to stimuli both decrease when they are not allowed to sleep. We all have something in common with fruit flies!

They're half asleep

During normal sleep, animals (and people) close both eyes, and both sides of the brain slow down. To remain mindful without compromising on sleep, many birds, some aquatic mammals (including dolphins, whales, and manatees), and possibly lizards have developed the capacity for unihemispheric sleep. They sleep with half their brain at a time, while the other half is awake, and only the eye opposite the brain's sleeping hemisphere is shut. In other words, when the right half of the brain sleeps, the left eye is shut, and they right eye stays open to scan the surroundings.

Ducks avoid danger through unihemispheric sleep
image: The Straight Dope

For reasons not fully known, terrestrial mammals don't do this, so scientists suspect that the cost of keeping half the brain awake may not be worth the benefits to most mammals and that sleeping with both halves of the brain is more efficient.

photo: Arnaud 25, Wikimedia Commons
So why, then, do birds and aquatic mammals use just half? Think of the benefits --- if you had to fly long distances or you lived under water but had to swim up to the surface to breathe every 10-20 minutes, you might develop this ability too!

Birds that migrate have to keep moving while simultaneously avoiding both getting lost and getting attacked by eagles.

Cetaceans (dolphins and whales) and manatees must be able to control their breathing while asleep, find their way to the surface, know when they are there, and stay in tune with others in their pod.

An alternative to unihemispheric sleep is seen in among blue tits (like chickadees for you North Americans) in Germany. These small birds slept in nest boxes for 10-14 hours per night, falling asleep at sunset and waking up just before sunrise, depending on the season, but they didn't sleep straight through the night. They typically woke up, stretched, preened, and even moved around more than 30 and up to 230 times per night.

Are there animals that don't sleep?

Is sleep just useful?  Or is it essential?

As one scientific study puts it: "What if sleep is not required but rather a kind of extreme indolence that animals indulge in when they have no more pressing needs, such as eating or reproducing? In many circumstances sleeping may be a less dangerous choice than roaming around, wasting energy and exposing oneself to predators."

There may be a few animals that appear not to need to fall asleep and may just slow down for parts of the day - this table lists potentially non-sleeping animals (the SD means Sleep Deprivation), though the evidence for their lack of actual sleep behavior tends to be the animals' continued reactions to experimental shocks or sleep deprivation, so the conclusions are still uncertain.

hippos get their beauty rest during the day
photo: Al Ain wildlife park
For example, we and other species that are presumed to sleep can and often do make up for lost sleep by sleeping longer, more deeply, and with fewer awakenings once we get the chance.

Furthermore, the development of unihemispheric sleep (remember? sleeping with half the brain while keeping the other half awake) in many animals suggests that getting at least some sleep is important for biological fitness for most animals.

Finally, both vertebrates and invertebrates suffer from sleep deprivation -- we all need to sleep to some extent to allow our brains and bodies to functional optimally.


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