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Why Do We Sleep?

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Why do we sleep?

Paul King:

A number of proposals and perspectives have emerged that, taken together, paint a compelling and converging picture for why sleep evolved and why it is now needed.

WHY SLEEP EVOLVED

The earliest differentiation into day versus night behavior in animals was probably driven by differences in temperature and the light available for vision. Day and night are further differentiated by the aggregate "behavior" of the ecosystem (other animals also have day/night cycles). As a result, the optimal day and night survival strategies are different, and animals would have adapted to synchronize their survival strategy to the 24-hour cycle.

Evidence is accumulating that complex nervous systems, and especially the brain, perform and benefit from internal maintenance activities. Some of these maintenance activities, such as "synaptic network stabilization," occur at the cellular level. Others, such as memory consolidation or the proposed memory transfer between brain areas, occur at the whole brain level.

The brain's internal maintenance disrupts optimal behavioral responsiveness. An animal can't be fully vigilant for predators while its brain is doing internal housekeeping. For example, memory consolidation requires information unrelated to the present moment to be accurately moved around the brain under the management of organized processes. Certainly the groggy state upon waking up is not optimal for fending off an attacker. For this reason, there is evolutionary pressure for the animal to postpone internal maintenance activities while engaging with the external environment (finding food, mates, etc.) so as to reduce risk of harm.

These postponed maintenance activities have to happen sometime. To optimize survival, the organism schedules a time for them when it will be safest, which includes finding a protected place to be and initiating all maintenance activities in parallel to get them out of the way. This leads to the temporal synchronization of all postponed maintenance activities.

Collectively, these four environmental and biological pressures lead to a bifurcation of external interactive behavior and internal maintenance activity that is synchronized with the 24-hour day/night cycle. They further encourage all postponed internal maintenance activities to be synchronized and performed in parallel—in other words, sleep.

Given the different day versus night ecosystems, it is natural that animals that have a survival edge during one phase of the 24-hour cycle would use the other phase for sleeping. "Diurnal" animals are specialized for engaging with the daytime environment, whereas "nocturnal" animals (cats, kangaroos, and owls, for example) are specialized for night and sleep during the day. However there are other patterns as well, such as "crepuscular" animals that are active at the day/night boundary (dawn or dusk).

WHY SLEEP IS NOW NEEDED

Given that a regular sleep period has formed, the question is: What goes on during sleep that is so important?

There appears to be no one single sleep activity that is the reason for sleep. Because the sleep/wake division of the 24-hour activity cycle has been around in animals for a long time—all known animals have a quiescent period—the sleep period has had hundreds of millions of years to acquire multiple uses.

At the cellular level, removing toxic radicals and strengthening or rebuilding tissues has been suggested.

In the brain, a number of uses for sleep have been identified or proposed [1], including:

  1. Restoring neurons biochemically
  2. Rescaling the connection strengths of synapses in the brain's neural networks to facilitate easier learning the next day. This might include rewiring activity (growing synapses).
  3. Consolidating (reorganizing and restructuring) memories. [3]
  4. Transferring memories from the memory-specialized fast-learning brain area (the hippocampus) to the higher-capacity more cognitively powerful area (the cerebral cortex). [4]

"At the whole animal behavioral level, the functions of sleep seem clear: energy is saved, performance is restored, and (in humans) affect becomes more positive. Such findings have led to the universal acknowledgement that sleep restores brain function." [1]

SLEEP AS A DIVERSE AND DECENTRALIZED PHENOMENON

A wide variety in sleep patterns can be found across animal species. Some animals (insects, fish) are quiescent but not fully "asleep" (inactive). In cetaceans (whales and dolphins), half of the brain goes to sleep at a time for one to two hours on an irregular schedule throughout the day and night. Bears, bats, and some rodents hibernate, which is a quiescent extended-sleep period lasting for weeks or months in the winter. Siegel (2008) reviews the phenomenon of sleep across animal species. [2]

Krueger et al (2008) reviews the functions and mechanisms of sleep in the brain and proposes that sleep is not a centralized process of the brain, as is normally thought, but is instead a decentralized intrinsic process of neural tissue. They propose that neural tissue initiates sleep onset in a decentralized fashion that propagates until it takes over the whole brain, in something like a social mass movement. The evolutionary pressure to partition activity into day/night divisions is also discussed. [1]

This chart shows the evolution of sleep in relation to the environment, metabolism, and the development of complex nervous systems. "R-A cycles" = Rest-Activity cycles. Sleep appears in the bottom two frames.


from Krueger et al (2008) [1]

This post originally appeared on Quora. Click here to view.

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Big Questions
What Does the Sergeant at Arms Do?
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
Chip Somodevilla, Getty Images

In 1981, shortly after Howard Liebengood was elected the 27th Sergeant at Arms of the United States Senate, he realized he had no idea how to address incoming president-elect Ronald Reagan on a visit. “The thought struck me that I didn't know what to call the President-elect,'' Liebengood told The New York Times in November of that year. ''Do you call him 'President-elect,' 'Governor,' or what?” (He went with “Sir.”)

It would not be the first—or last—time someone wondered what, exactly, a Sergeant at Arms (SAA) should be doing. Both the House and the Senate have their own Sergeant at Arms, and their visibility is highest during the State of the Union address. For Donald Trump’s State of the Union on January 30, the 40th Senate SAA, Frank Larkin, will escort the senators to the House Chamber, while the 36th House of Representatives SAA, Paul Irving, will introduce the president (“Mister [or Madam] Speaker, the President of the United States!”). But the job's responsibilities extend far beyond being an emcee.

The Sergeants at Arms are also their respective houses’ chief law enforcement officers. Obliging law enforcement duties means supervising their respective wings of the Capitol and making sure security is tight. The SAA has the authority to find and retrieve errant senators and representatives, to arrest or detain anyone causing disruptions (even for crimes such as bribing representatives), and to control who accesses chambers.

In a sense, they act as the government’s bouncers.

Sergeant at Arms Frank Larkin escorts China's president Xi Jinping
Senat Sergeant at Arms Frank Larkin (L) escorts China's president Xi Jinping during a visit to Capitol Hill.
Astrid Riecken, Getty Images

This is not a ceremonial task. In 1988, Senate SAA Henry Giugni led a posse of Capitol police to find, arrest, and corral Republicans missing for a Senate vote. One of them, Republican Senator Bob Packwood of Oregon, had to be carried to the Senate floor to break the filibustering over a vote on senatorial campaign finance reform.

While manhandling wayward politicians sounds fun, it’s more likely the SAAs will be spending their time on administrative tasks. As protocol officer, visits to Congress by the president or other dignitaries have to be coordinated and escorts provided; as executive officer, they provide assistance to their houses of Congress, with the Senate SAA assisting Senate offices with computers, furniture, mail processing, and other logistical support. The two SAAs also alternate serving as chairman of the Capitol Police board.

Perhaps a better question than asking what they do is pondering how they have time to do it all.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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