Why Do We Sleep ?

Hunger and Eating; Sleepiness and Sleep

As with eating well, good sleep is a staple of optimal health.

While we may not often think about why we sleep, most of us acknowledge at some level that sleep makes us feel better. We feel more alert, more energetic, happier, and better able to function following a good night of sleep. However, the fact that sleep makes us feel better and that going without sleep makes us feel worse only begins to explain why sleep might be necessary. 

One way to think about the function of sleep is to compare it to another of our life-sustaining activities: eating. Hunger is a protective mechanism that has evolved to ensure that we consume the nutrients our bodies require to grow, repair tissues, and function properly. And although it is relatively easy to grasp the role that eating serves— given that it involves physically consuming the substances our bodies need—eating and sleeping are not as different as they might seem.

Both eating and sleeping are regulated by powerful internal drives. Going without food produces the uncomfortable sensation of hunger, while going without sleep makes us feel overwhelmingly sleepy. And just as eating relieves hunger and ensures that we obtain the nutrients we need, sleeping relieves sleepiness and ensures that we obtain the sleep we need. Still, the question remains: Why do we need sleep at all? Is there a single primary function of sleep, or does sleep serve many functions?

An Unanswerable Question?

Scientists have explored the question of why we sleep from many different angles. They have examined, for example, what happens when humans or other animals are deprived of sleep. In other studies, they have looked at sleep patterns in a variety of organisms to see if similarities or differences among species might reveal something about sleep's functions. Yet, despite decades of research and many discoveries about other aspects of sleep, the question of why we sleep has been difficult to answer.

The lack of a clear answer to this challenging question does not mean that this research has been a waste of time. In fact, we now know much more about the function of sleep, and scientists have developed several promising theories to explain why we sleep. In light of the evidence they have gathered, it seems likely that no single theory will ever be proven correct. Instead, we may find that sleep is explained by two or more of these explanations. The hope is that by better understanding why we sleep, we will learn to respect sleep's functions more and enjoy the health benefits it affords.

This essay outlines several current theories of why we sleep. To learn more about them, be sure to check out the "Bookshelf" feature at the end of this essay. There you'll find links to articles by researchers who are studying this fascinating question.

Theories of Why We Sleep

Inactivity Theory

Arctic Fox at rest.

One of the earliest theories of sleep, sometimes called the adaptive or evolutionary theory, suggests that inactivity at night is an adaptation that served a survival function by keeping organisms out of harm’s way at times when they would be particularly vulnerable. The theory suggests that animals that were able to stay still and quiet during these periods of vulnerability had an advantage over other animals that remained active. These animals did not have accidents during activities in the dark, for example, and were not killed by predators. Through natural selection, this behavioral strategy presumably evolved to become what we now recognize as sleep.

A simple counter-argument to this theory is that it is always safer to remain conscious in order to be able to react to an emergency (even if lying still in the dark at night). Thus, there does not seem to be any advantage of being unconscious and asleep if safety is paramount.

Energy Conservation Theory

Although it may be less apparent to people living in societies in which food sources are plentiful, one of the strongest factors in natural selection is competition for and effective utilization of energy resources. The energy conservation theory suggests that the primary function of sleep is to reduce an individual’s energy demand and expenditure during part of the day or night, especially at times when it is least efficient to search for food.

Lions conserving energy after a meal.

Research has shown that energy metabolism is significantly reduced during sleep (by as much as 10 percent in humans and even more in other species). For example, both body temperature and caloric demand decrease during sleep, as compared to wakefulness. Such evidence supports the proposition that one of the primary functions of sleep is to help organisms conserve their energy resources. Many scientists consider this theory to be related to, and part of, the inactivity theory.

Restorative Theories

Another explanation for why we sleep is based on the long-held belief that sleep in some way serves to "restore" what is lost in the body while we are awake. Sleep provides an opportunity for the body to repair and rejuvenate itself. In recent years, these ideas have gained support from empirical evidence collected in human and animal studies. The most striking of these is that animals deprived entirely of sleep lose all immune function and die in just a matter of weeks. This is further supported by findings that many of the major restorative functions in the body like muscle growth, tissue repair, protein synthesis, andgrowth hormone release occur mostly, or in some cases only, during sleep. 

Other rejuvenating aspects of sleep are specific to the brain andcognitive function. For example, while we are awake, neurons in the brain produce adenosine, a by-product of the cells' activities. The build-up of adenosine in the brain is thought to be one factor that leads to our perception of being tired. (Incidentally, this feeling is counteracted by the use of caffeine, which blocks the actions of adenosine in the brain and keeps us alert.) Scientists think that this build-up of adenosine during wakefulness may promote the "drive to sleep." As long as we are awake, adenosine accumulates and remains high. During sleep, the body has a chance to clear adenosine from the system, and, as a result, we feel more alert when we wake.

Brain Plasticity Theory

PET scan showing brain activity in a 20-year-old.

One of the most recent and compelling explanations for why we sleep is based on findings that sleep is correlated to changes in the structure and organization of the brain. This phenomenon, known as brain plasticity, is not entirely understood, but its connection to sleep has several critical implications. It is becoming clear, for example, that sleep plays a critical role in brain development in infants and young children. Infants spend about 13 to 14 hours per day sleeping, and about half of that time is spent in REM sleep, the stage in which most dreams occur. A link between sleep and brain plasticity is becoming clear in adults as well. This is seen in the effect that sleep and sleep deprivation have on people's ability to learn and perform a variety of tasks.

Although these theories remain unproven, science has made tremendous strides in discovering what happens during sleep and what mechanisms in the body control the cycles of sleep and wakefulness that help define our lives. While this research does not directly answer the question, "Why do we sleep?" it does set the stage for putting that question in a new context and generating new knowledge about this essential part of life.This theory and the role of sleep in learning are covered in greater detail in Sleep, Learning, and Memory.

Eye

Eyes are organs of the visual system. They provide organisms vision, the ability to process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complexoptical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.^[1] Image-resolving eyes are present in molluscs, chordates and arthropods.

The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for theentrainment of circadian rhythms.^[3] From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to thesuprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex.

Aircraft

An aircraft is a machine that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using thedynamic lift of an airfoil,^[1] or in a few cases the downward thrust from jet engines.

The human activity that surrounds aircraft is called aviation. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may beremotely controlled or self-controlled by onboard computers. Aircraft may be classified by different criteria, such as lift type, aircraft propulsion, usage and others.

Rocket

A rocket (from Italian rocchetto "bobbin")^{[nb 1][1]} is a missile, spacecraft, aircraft or other vehicle that obtains thrust from a rocket engine. Rocket engine exhaust is formed entirely from propellant carried within the rocket before use.^[2] Rocket engines work by action and reaction and push rockets forward simply by expelling their exhaust in the opposite direction at high speed, and can therefore work in the vacuum of space.

In fact, rockets work more efficiently in space than in an atmosphere. Multi-stage rockets are capable of attaining escape velocity from Earth and therefore can achieve unlimited maximum altitude. Compared with airbreathing engines, rockets are lightweight and powerful and capable of generating large accelerations. To control their flight, rockets rely on momentum, airfoils, auxiliary reaction engines, gimballed thrust, momentum wheels, deflection of the exhaust stream, propellant flow, spin, and/or gravity.

Rockets for military and recreational uses date back to at least 13th century China.^[3] Significant scientific, interplanetary and industrial use did not occur until the 20th century, when rocketry was the enabling technology for the Space Age, including setting foot on the moon. Rockets are now used for fireworks, weaponry, ejection seats,launch vehicles for artificial satellites, human spaceflight, and space exploration.

Chemical rockets are the most common type of high power rocket, typically creating a high speed exhaust by the combustion of fuel with an oxidizer. The stored propellant can be a simple pressurized gas or a single liquid fuel that disassociates in the presence of a catalyst (monopropellants), two liquids that spontaneously react on contact (hypergolic propellants), two liquids that must be ignited to react, a solid combination of fuel with oxidizer (solid fuel), or solid fuel with liquid oxidizer (hybrid propellant system). Chemical rockets store a large amount of energy in an easily released form, and can be very dangerous. However, careful design, testing, construction and use minimizes risks.

Weighing scale

Weighing scales (or weigh scales or scales) are devices to measure weight or calculate mass. Spring balances or spring scales measure weight (force) by balancing the force due to gravity against the force on a spring, whereas a balance or pair of scales using a balance beam compares masses by balancing the weight due to the mass of an object against the weight of a known mass or masses. Either type can be calibrated to read in units of force such as newtons, or in units of mass such as kilograms, but the balance or pair of scales using a traditional balance beam to compare masses will read correctly for mass even if moved to a place with a different (non-zero) gravitational field strength (but would then not read correctly if calibrated in units of force), while the spring balance would read correctly in force in a different gravitational field strength (but would not read correctly if calibrated in units of mass).

Paper money, a Chinese invention?

First paper, then paper money. This is pure logic. It is hardly surprising that the first notes or better, the first paper money, appeared in China. With the invention of paper and printing on its account, this country was almost destined to produce the first paper money.

For centuries the mulberry tree has been cultivated in the Valley of the Yellow River (Shang period, 18th to 12th century BC). The first traces of paper date back to the 2nd half of the 1st century BC but then it was not used as writing material. For their traditional calligraphy with brushes they used linen, hemp, bamboo (cane) and bark of the mulberry tree. Important progress has been made between the 2nd and 4th century AD: Thanks to the use of soaked bast of the mulberry the quality of the pulp significantly improved and paper became less heavy. The improvement was such that paper gradually replaced the former bamboomats. Clerical texts and reports for the Court were henceforth written on paper but still in a vertical direction. This centuries-old way of writing is probably a result of writing on strips of bamboo which were tied together.

From paper to paper money

Paper fabrication during the Han Dynasty (206 BC – 220 AD)

During the Tang Dynasty (618-907) there was a growing need of metallic currency, but thanks to the familiarity with the idea of credit the Chinese were ready to accept pieces of paper or paper drafts. This practice is derived from the credit notes used by merchants for their long-distance trade.

Due to this lack of coins, also the dead had to change their habits of taking a coin with them to pay their passage to the other world. About the 6th century notes replaced coins as burial money. May we consider this as a real means of payment? Of course not, but it is remarkable that also here paper replaces very smoothly the copper coins that were used before.

At the end of the Tang period, traders deposited their values with their corporations. In exchange, they received bearer notes or the so-called hequan. Those hequan were a real success and the idea was exploited by the Authorities. Merchants were invited to deposit henceforth their metallic money in the Government Treasury in exchange for official “compensation notes”, called Fey-thsian or flying money.

During the Song Dynasty (960-1276) booming business in the region of Tchetchuan likewise resulted in a shortage of copper money. Some merchants issued private drafts covered by a monetary reserve which initially consisted of coins and salt, later of gold and silver. Those notes are considered to be the first to circulate as legal tender. In 1024 the Authorities confer themselves the issuing monopoly and under Mongol governement, during the Yuan Dynasty (1279-1367), paper money becomes the only legal tender. During the Ming Dynasty (1368-1644) the issuing of notes is conferred to the Ministry of Finance.

On all notes issued between 1380 (13th year of the Emperor Hung Wu’s reign) and 1560 the names of Hung Wu and the Minister of Finance can be read. Those notes were issued in values of 100, 200, 300, 400 and 500 wen and 1 kuan or 1000 wen. Thus one kuan had the same value as 1000 copper coins or 1 liang (1 tael) of silver; 4 kuan equalled 1 liang of gold.

Unfortunately, those notes were issued continuously without redrawing from circulation the old ones. This practice of course led to an inflationary spiral: in the beginning, in 1380, one guan was worth 1000 copper coins, in 1535, one guan valued merely 0,28 copper coin!

Note printed on dark slate paper of the mulberry tree

The note in the showcase dates back to the same period. It is fairly large (340 x 221 mm) and printed on dark slate paper of the mulberry tree. The inscriptions in the heading read the name of the issuer and the name Hung Wu, the founder of the Dynasty. The value of Yi guan (or 1 kuan) is represented by the two characters and by ten heaps of coins just beneath them. The inscriptions left and right of the value there read as Note of the great Ming dynasty and Circulating within the Empire.

In the inferior cartouche, a long inscription, which must be read vertically from right to left, gives more information on the note : Printed by imperial authorisation by the Minister of Finance (2 columns to the right); The note of the Great Ming Dynasty circulates together with copper money. Forgers will be decapitated and those who can give information which leads to the arrest of forgers are offered a reward of 250 liang of silver on top of the belongings of the forger (4 central columns); (made in the era) Hung Wu ___year ___month __ day (left column). The blanks had to be filled in by hand but this was never put into practice. As a result the name “Hung Wu” figures on the notes even after the end of his reign. The borders are attractively designed with arabesque style flowers and dragons.

The discovery of Chinese paper money in Europe

In the travel accounts of the Venetian traveller Marco Polo the reader becomes familiar with the fascinating world of paper money production. This money has been put into circulation during the Yuan period by the Mongol chief Kublai Khan (1214-1294) : “It is in the city of Khanbalik that the Great Khan possesses his Mint. (…) In fact, paper money is made there from the sapwood of the mulberry tree, whose leaves feed the silk worm. The sapwood, between the bark and the heart, is extracted, ground and then mixed with glue and compressed into sheets similar to cotton paper sheets, but completely black. (…) The method of issue is very formal, as if the substance were pure gold or silver. On each sheet which is to become a note, specially appointed officials write their name and affix their seal. When this work has been done in accordance with the rules, the chief impregnates his seal with pigment and affixes his vermillion mark at the top of the sheet. That makes the note authentic. This paper currency is circulated in every part of the Great Khan’s dominions, nor dares any person, at the peril of his life, refuse to accept it in payment.”

Triangular trade

Triangular trade or triangle trade is a historical term 

 indicating trade among three ports or regions. Triangular

trade usually evolves when a region has export commodities that are not required in the region from which its major imports come. Triangular trade thus provides a method for rectifying trade imbalances between the above regions.

The particular routes were historically also shaped by the powerful influence of winds and currents during the age of sail. For example, from the main trading nations of Western Europe it was much easier to sail westwards after first going south of 30 N latitude and reaching the so-called "trade winds"; thus arriving in the Caribbean rather than going straight west to the North American mainland. Returning from North America, it is easiest to follow the Gulf Stream in a northeasterly direction using the westerlies. A similar triangle to this, called the volta do mar was already being used by the Portuguese, before Christopher Columbus' voyage, to sail to the Canary Islands and the Azores. Columbus simply expanded the triangle outwards, and his route became the main way for Europeans to reach, and return from, the Americas.

First use of numbers

Bones and other artifacts have been discovered with marks cut into them that many believe are tally marks.^[10] These tally marks may have been used for counting elapsed time, such as numbers of days, lunar cycles or keeping records of quantities, such as of animals.

A tallying system has no concept of place value (as in modern decimal notation), which limits its representation of large numbers. Nonetheless tallying systems are considered the first kind of abstract numeral system.

The first known system with place value was the Mesopotamian base 60 system (ca. 3400 BC) and the earliest known base 10 system dates to 3100 BC in Egypt.

Zero:

The use of 0 as a number should be distinguished from its use as a placeholder numeral in place-value systems. Many ancient texts used 0. Babylonian (Modern Iraq) and Egyptian texts used it. Egyptians used the word nfr to denote zero balance in double entry accounting entries. Indian texts used a Sanskrit word Shunyeor shunya to refer to the concept of void. In mathematics texts this word often refers to the number zero.^[12]

Records show that the Ancient Greeks seemed unsure about the status of 0 as a number: they asked themselves "how can 'nothing' be something?" leading to interesting philosophical and, by the Medieval period, religious arguments about the nature and existence of 0 and the vacuum. The paradoxes of Zeno of Eleadepend in large part on the uncertain interpretation of 0. (The ancient Greeks even questioned whether 1 was a number.)

The late Olmec people of south-central Mexico began to use a true zero (a shell glyph) in the New World possibly by the 4th century BC but certainly by 40 BC, which became an integral part of Maya numerals and the Maya calendar. Mayan arithmetic used base 4 and base 5 written as base 20. Sanchez in 1961 reported a base 4, base 5 "finger" abacus.

By 130 AD, Ptolemy, influenced by Hipparchus and the Babylonians, was using a symbol for 0 (a small circle with a long overbar) within a sexagesimal numeral system otherwise using alphabetic Greek numerals. Because it was used alone, not as just a placeholder, this Hellenistic zero was the first documented use of a true zero in the Old World. In laterByzantine manuscripts of his Syntaxis Mathematica (Almagest), the Hellenistic zero had morphed into the Greek letter omicron (otherwise meaning 70).

Another true zero was used in tables alongside Roman numerals by 525 (first known use by Dionysius Exiguus), but as a word, nulla meaning nothing, not as a symbol. When division produced 0 as a remainder, nihil, also meaning nothing, was used. These medieval zeros were used by all future medieval computists (calculators of Easter). An isolated use of their initial, N, was used in a table of Roman numerals by Bede or a colleague about 725, a true zero symbol.

An early documented use of the zero by Brahmagupta (in the Brāhmasphuṭasiddhānta) dates to 628. He treated 0 as a number and discussed operations involving it, including division. By this time (the 7th century) the concept had clearly reached Cambodia as Khmer numerals, and documentation shows the idea later spreading to China and the Islamic world.

Number

A number is a mathematical object used to count, measure, and label.^{[citation needed]} The original examples are the natural numbers 1, 2, 3, and so forth. A notational symbol that represents a number is called a numeral. In addition to their use in counting and measuring, numerals are often used for labels (as withtelephone numbers), for ordering (as with serial numbers), and for codes (as with ISBNs). In common usage, number may refer to a symbol, a word, or a mathematical abstraction.

In mathematics, the notion of number has been extended over the centuries to include 0, negative numbers, rational numbers such as ${\displaystyle {\frac {1}{2}}}$ and ${\displaystyle -{\frac {2}{3}}}$, real numberssuch as ${\displaystyle {\sqrt {2}}}$ and ${\displaystyle \pi }$, complex numbers, which extend the real numbers by including ${\displaystyle {\sqrt {-1}}}$, and sometimes additional objects. Calculations with numbers are done with arithmetical operations, the most familiar being addition, subtraction, multiplication, division, and exponentiation. Their study or usage is called arithmetic. The same term may also refer to number theory, the study of the properties of the natural numbers.

Besides their practical uses, numbers have cultural significance throughout the world.^[1][2] For example, in Western society the number 13 is regarded as unlucky, and "a million" may signify "a lot."^[1] Though it is now regarded as pseudoscience, numerology, the belief in a mystical significance of numbers permeated ancient and medieval thought.^[3] Numerology heavily influenced the development of Greek mathematics, stimulating the investigation of many problems in number theory which are still of interest today.^[3]

During the 19th century, mathematicians began to develop many different abstractions which share certain properties of numbers and may be seen as extending the concept. Among the first were thehypercomplex numbers, which consist of various extensions or modifications of the complex number system. Today, number systems are considered important special examples of much more general categories such as rings and fields, and the application of the term "number" is a matter of convention, without fundamental significance.

Air Composition

Dry air is a mechanical mixture of nitrogen, oxygen, carbon dioxide and more

Air is a mixture of gases - 78% nitrogen and 21% oxygen - with traces of water vapor, carbon dioxide, argon, and various other components.

Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.

Gas	Ratio compared to Dry Air (%)		Molecular Mass - M -(kg/kmol)	Chemical Symbol	Boiling Point
	By volume	By weight			(K)	(oC)
Oxygen	20.95	23.20	32.00	O2	90.2	-182.95
Nitrogen	78.09	75.47	28.02	N2	77.4	-195.79
Carbon Dioxide	0.03	0.046	44.01	CO2	194.7	-78.5
Hydrogen	0.00005	~ 0	2.02	H2	20.3	-252.87
Argon	0.933	1.28	39.94	Ar	84.2	-186
Neon	0.0018	0.0012	20.18	Ne	27.2	-246
Helium	0.0005	0.00007	4.00	He	4.2	-269
Krypton	0.0001	0.0003	83.8	Kr	119.8	-153.4
Xenon	9 10-6	0.00004	131.29	Xe	165.1	-108.1

• The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
• The composition of air is unchanged until elevation of approximately 10.000 m
• The average air temperature diminishes at the rate of 0.6^oC for each 100 m vertical height
• "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0^oC sea level and at standard gravity (32.174 ft/sec²)

Other components in air

• Sulfur dioxide - SO₂ - 1.0 parts/million (ppm)
• Methane - CH₄ - 2.0 parts/million (ppm)
• Nitrous oxide - N₂O - 0.5 parts/million (ppm)
• Ozone - O₃ - 0 to 0.07 parts/million (ppm)
• Nitrogen dioxide - NO₂ - 0.02 parts/million (ppm)
• Iodine - I₂ - 0.01 parts/million (ppm)
• Carbon monoxide - CO - 0 to trace (ppm)
• Ammonia - NH₃ - 0 to trace (ppm)

Common Pressure Units frequently used as alternative to "one Atmosphere"

• 76 Centimeters (760 mm) of Mercury
• 29.921 Inches of Mercury
• 10.332 Meters of Water
• 406.78 Inches of Water
• 33.899 Feet of Water
• 14.696 Pound-Force per Square Inch
• 2116.2 Pounds-Force per Square Foot
• 1.033 Kilograms-Force per Square Centimeter
• 101.33 kiloPascal