How much of the human body is processed sunlight?

How much of the human body is processed sunlight?

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So I believe (and correct me if I'm wrong--it's logic, not research) that the human body is pretty much all just processed sunlight based on the food chain. How much, if any, of the human body is made up of materials that did not originate from the sun (e.g., various minerals or other trace elements)?

I think that Carl Sagan said it best:

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

The current view of cosmology is that every atom that isn't hydrogen, helium, or lithium was created in the fusion furnace in the hearts of giant stars. These heavy atoms were then spread about the place by supernovas.

However, there's a lot more to the matter that makes up living creatures than just the atoms. The vast majority of the free energy that allows for the creation of intricate organic molecules with complex patterns of chemical bonds (e.g. protein, DNA, etc.) does indeed originate with our sun.


Humans (Homo sapiens) are the most abundant and widespread species of primates, characterized by bipedality and large complex brains enabling the development of advanced tools, culture and language. Humans are highly social beings and tend to live in large complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established a wide variety of values, social norms, and rituals, which bolster human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, philosophy, mythology, religion, and other fields of knowledge.

Humans evolved from other hominins in Africa several million years ago. Although some scientists equate humans with all members of the genus Homo, in common usage it generally refers to Homo sapiens, the only extant member. H. sapiens emerged around 300,000 years ago, evolving from Homo erectus and migrating out of Africa, gradually replacing local populations of archaic humans. Early humans were hunter-gatherers, before settling in the Fertile Crescent and other parts of the Old World. Access to food surpluses led to the formation of permanent human settlements and the domestication of animals. As populations became larger and denser, forms of governance developed within and between communities and a number of civilizations rose and fell. Humans have continued to expand, with over 7.8 billion humans occupying almost all regions of the world in 2021.

Genes and the environment influence human biological variation in visible characteristics, physiology, disease susceptibility, mental abilities, body size and life span. Though humans vary in many traits (such as genetic predispositions and physical features), two humans on average are over 99% similar, with the most genetically diverse populations from Africa. The greatest degree of genetic variation exists between males and females. On average, men have greater body strength and women generally have a higher body fat percentage. Females undergo menopause and become infertile decades before the end of their lives. They also have a longer life span in almost every population around the world. The division into male and female gender roles has varied historically, and challenges to predominant gender norms have recurred in many societies.

Humans are omnivorous, capable of consuming a wide variety of plant and animal material, and have used fire to prepare and cook food since the time of H. erectus. They can survive for up to eight weeks without food, and three or four days without water. Humans are generally diurnal, sleeping on average seven to nine hours per day. Childbirth is dangerous, with a high risk of complications and death. Both the mother and the father provide care for human offspring who are helpless at birth.

Humans have a large and highly developed prefrontal cortex, the region of the brain associated with higher cognition. They are intelligent beings, capable of episodic memory, flexible facial expressions, self-awareness and a theory of mind. The human mind is capable of introspection, private thought, imagination, volition and forming views on existence. This has allowed great technological advancements and complex tool development possible through reason and the transmission of knowledge to future generations. Language, art and trade are defining characteristics of humans. Long-distance trade routes might have led to cultural explosions and resource distribution that gave humans an advantage over other similar species.

How Much of Your Body Is Water?

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    • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
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    Have you ever wondered how much of your body is water? The percentage of water varies according to your age and gender. Here's a look at how much water is inside you.

    The amount of water in the human body ranges from 45-75%.   The average adult human body is 50-65% water, averaging around 57-60%. The percentage of water in infants is much higher, typically around 75-78% water, dropping to 65% by one year of age.  

    Body composition varies according to gender and fitness level because fatty tissue contains less water than lean tissue. The average adult male is about 60% water. The average adult woman is about 55% water because women naturally have more fatty tissue than men.   Overweight men and women have less water, as a percent than their leaner counterparts.

    Who Has the Most Water?

    • Babies and children have the highest percentage of water.
    • Adult men contain the next highest level of water.
    • Adult women contain a lower percentage of water than babies or men.
    • Obese men and women have less water, as a percentage than lean adults.

    The percent of water depends on your hydration level.   People feel thirsty when they have already lost around 2-3% of their body's water. Being dehydrated by just 2% impairs performance in mental tasks and physical coordination.  

    Although liquid water is the most abundant molecule in the body, additional water is found in hydrated compounds. About 30-40% of the weight of the human body is the skeleton, but when the bound water is removed, either by chemical desiccation or heat, half the weight is lost.  

    Positive UV Radiation Effects on Humans

    A few positive ultraviolet light effects for humans are worth mentioning. The main one of these is the ability of UV light (specifically UV-A) to trigger the production of vitamin D by our bodies. This is needed for the bones, muscles and the immune system, and it is suspected to lower the risk of colon cancer.

    UV light also has beneficial effects on skin conditions like psoriasis because it slows the growth of skin cells and thereby reduces symptoms. Sunlight exposure (i.e., UV exposure) also stimulates the production of tryptamines, which improve mood.

    What happens if I have too much melatonin?

    There are large variations in the amount of melatonin produced by individuals and these are not associated with any health problems. The main consequences of swallowing large amounts of melatonin are drowsiness and reduced core body temperature. Very large doses have effects on the performance of the human reproductive system. There is also evidence that very high concentrations of melatonin have an antioxidant effect, although the purpose of this has not yet been established.


    Heat is continuously generated in the human body by metabolic processes and exchanged with the environment and among internal organs by conduction, convection, evaporation and radiation. Transport of heat by the circulatory system makes heat transfer in the body — or bioheat transfer — a specific branch of this general science. As in all entities, the principle of conservation of energy yields:

    where the terms denote, from left to right, the rate of heat generation due to metabolic processes rate of heat stored in body tissues and fluids heat lost to the environment and adjacent tissues and the rate of work performed by the tissue. This latter quantity is usually negligible at the tissue level.

    In the tissue element shown in Figure 1, heat due to metabolic processes (5–10,000 W/m 3 ) is generated at a variable rate, which, when integrated over the entire control volume, obtains:

    where denotes the spatial coordinate and t is time.

    Figure 1. Control volume of tissue element.

    Under unsteady conditions, part of the heat flow will be stored in the control volume:

    where ρ is tissue density (900–1600) kg/m 3 ) c is tissue specific heat (2.1–3.8 kJ/kgK) and T is tissue temperature.

    The term representing heat lost to adjacent tissues and to the environment contains a number of components, one of which is heat exchanged by diffusion (Fourier’s Law of conduction):

    where k is the thermal conductivity of the tissue (0.29–1.06 W/mK) T is tissue temperature gradient is outward-pointing unit vector and A is control volume surface area.

    A second component of the heat lost to adjacent tissues is due to blood perfusion. Blood circulates in a variety of vessels ranging in lumen diameter from the 2.5 cm aorta to the 6–10 mm capillaries. Due to this four-fold size distribution, heat transport effects of blood are coupled to the specific group of vessels under consideration. A common approach to modeling this effect is to assume that the rate of heat taken up by the circulating blood at the capillary level equals the difference between the venous and arterial temperatures times the flow rate (Fick’s Law):

    where rbcb is blood heat capacity (≈4000 kJ/m 3 K) and wb is volumetric blood perfusion rate (0.17–50 kg/m 3 s). At the capillary level, blood flow velocity is very slow and thermal equilibration with surrounding tissue occurs. Thus, Eq. (5) may be modified by setting Tv = T (Pennes, 1948). When all the terms are substituted into Eq. (1) and integrated over the entire volume and surface area, the well-known bioheat equation is obtained:

    Equation (6) has been very useful in the analysis of heat transfer in various body organs and tissues characterized by a dense capillary bed. Other thermal effects due to blood flow are not adequately accounted for by Eq. (6), including: 1) countercurrent heat transfer between adjacent vessels 2) directionality effects due to the presence of larger blood vessels and 3) heat exchange with larger vessels in which complete thermal equilibrium may not be assumed. These issues have been analyzed by Chen and Holmes (1980) and by Weinbaum et al. (1984).

    Heat is exchanged with the environment through a complex combination of conduction, convection, radiation and evaporation. Clothing worn by humans and natural integuments also play a role. As a good approximation, these effects may be calculated by an equation of the form:

    where is the amount of heat exchanged and hi is the heat exchange coefficient (2.3–2.7 W/m 2 K for free convection, 7.4V 0.67 for forced convection, where V is wind velocity, m/s, and 3.8–5.1 for radiation) is average body surface temperature To is environmental temperature and AD is Dubois’ body surface given by:

    where m is body mass in kg and h is height in m.


    Chen, M. M. and Holmes, K. R. (1980) Microvasculature Contributions in Tissue Heat Transfer, In: Thermal Characteristics of Tumors: Applications in Detection and Treatment , R. K. Jain and P. M. Gullino Eds., Ann. N.Y. Acad. Sci., 335:137–150.

    Pennes, H. H. (1948) Analysis of Tissue and Arterial Blood Temperatures in the Resting Human Forearm, J. Appl. Physiol. , 1:93–122.

    Shitzer, A. and Eberhart, R. C., Eds. (1985) Heat transfer in medicine and biology — analysis and applications , Plenum Press, New York.

    Weinbaum, S., Jiji, L. M., and Lemons, D. (1984) Theory and Experiment for the Effect of Vascular Microstructure on Surface Heat Transfer, ASME J. Biomech. Eng. , 106:321–330 (Pt. 1) 331–341 (Pt. 2).

    Chemical Digestion

    Chemical digestion is the biochemical process in which macromolecule s in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates , protein s , lipid s , and nucleic acids . Carbohydrates must be broken down into simple sugar s , proteins into amino acid s , lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine ( duodenum ).

    Digestive Enzymes

    Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze, or speed up, biochemical reactions. Digestive enzymes are secreted by exocrine gland s or by the mucosal layer of epithelium lining the gastrointestinal tract. In the mouth , digestive enzymes are secreted by salivary gland s . The lining of the stomach secretes enzymes, as does the lining of the small intestine . Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. The following table lists several important digestive enzymes, the organs and/or glands that secrete them, the compounds they digest, and the pH necessary for optimal functioning. You can read more about them below.

    Table 15.3.1: Digestive Enzymes
    Digestive Enzyme Source Organ Site of Action Reactant and Product Optimal pH
    Salivary Amylase Salivary Glands Mouth starch + water ⇒ maltose Neutral
    Pepsin Stomach Stomach protein + water ⇒ peptides Acidic
    Pancreatic Amylase Pancreas Duodenum starch + water ⇒ maltose Basic
    Maltase Small intestine Small intestine maltose + water ⇒ glucose Basic
    Sucrase Small intestine Small intestine sucrose + water ⇒ glucose + fructose Basic
    Lactase Small intestine Small intestine lactose + water ⇒ glucose + galactose Basic
    Lipase Pancreas Duodenum fat droplet and water ⇒ glycerol and fatty acids Basic
    Trypsin Pancreas Duodenum protein + water ⇒ peptides Basic
    Chymotrypsin Pancreas Duodenum protein + water ⇒ peptides Basic
    Peptidases Small intestine Small intestine peptides + water ⇒ Basic
    Deoxyribonuclease Pancreas Duodenum DNA + water ⇒ nucleotide fragments Basic
    Ribonuclease Pancreas Duodenum RNA + water ⇒ nucleotide fragments Basic
    Nuclease Small intestine Small intestine nucleic acids + water ⇒ nucleotide fragments Basic
    Nucleosidases Small intestine Small intestine nucleotides + water ⇒ nitrogen base + phosphate sugar Basic

    Chemical Digestion of Carbohydrates

    About 80% of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch . Additional dietary carbohydrates include the animal polysaccharide glycogen , along with some sugars, which are mainly disaccharide s .

    The process of chemical digestion for some carbohydrates is illustrated Figure 15.3.4. To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus — which lubricates the food — and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.

    Other sugars are digested with the help of different enzymes produced by the small intestine. Sucrose (or table sugar), for example, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose. Glucose and galactose are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.

    Figure 15.3.4 The process of chemical digestion for some carbohydrates.

    Chemical Digestion of Proteins

    Proteinsno post consist of polypeptides, which must be broken down into their constituent amino acid s before they can be absorbed. An overview of this process is shown in Figure 15.3.5. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin (secreted by the stomach), and trypsin and chymotrypsin (secreted by the pancreas). The stomach also secretes hydrochloric acid (HCl), making the contents highly acidic, which is a required condition for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline (basic) environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidase s , also secreted by the pancreas.

    Figure 15.3.5 Chemical digestion of proteins.

    Chemical Digestion of Lipids

    The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase , which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

    Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the liver . As shown in the diagram below (Figure 15.3.6), bile is required for the digestion of lipids, because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies (or breaks up) large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase, and also point the hydrophilic (“water-loving”) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

    Figure 15.3.6 Bile from the liver and lipase from the pancreas help digest lipids in the small intestine.

    Chemical Digestion of Nucleic Acids

    Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

    Bacteria in the Digestive System

    Your large intestine is not just made up of cells. It is also an ecosystem , home to trillions of bacteria known as the “gut flora” (Figure 15.3.7). But don’t worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth.

    Gut bacteria have several roles in the body. For example, intestinal bacteria:

    • Produce vitamin B12 and vitamin K.
    • Control the growth of harmful bacteria.
    • Break down poisons in the large intestine.
    • Break down some substances in food that cannot be digested, such as fibre and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach.

    Figure 15.3.7 Commensal (good) bacteria (shown in red) reside among the mucus (green) and epithelial cells (blue) of a small intestine.

    A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food.

    It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet — there are over 7 billion people on the planet!

    The bacteria in your digestive system are from anywhere between 300 and 1,000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the body’s immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.

    As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship.

    Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weigh just about two pounds.

    How much of the human body is processed sunlight? - Biology

    We spend a third of our lives doing it.

    Napoleon, Florence Nightingale and Margaret Thatcher got by on four hours a night.

    Thomas Edison claimed it was waste of time.

    So why do we sleep? This is a question that has baffled scientists for centuries and the answer is, no one is really sure. Some believe that sleep gives the body a chance to recuperate from the day's activities but in reality, the amount of energy saved by sleeping for even eight hours is miniscule - about 50 kCal, the same amount of energy in a piece of toast.

    We have to sleep because it is essential to maintaining normal levels of cognitive skills such as speech, memory, innovative and flexible thinking. In other words, sleep plays a significant role in brain development.

    What would happen if we didn't sleep?

    A good way to understand the role of sleep is to look at what would happen if we didn't sleep. Lack of sleep has serious effects on our brain's ability to function. If you've ever pulled an all-nighter, you'll be familiar with the following after-effects: grumpiness, grogginess, irritability and forgetfulness. After just one night without sleep, concentration becomes more difficult and attention span shortens considerably.

    With continued lack of sufficient sleep, the part of the brain that controls language, memory, planning and sense of time is severely affected, practically shutting down. In fact, 17 hours of sustained wakefulness leads to a decrease in performance equivalent to a blood alcohol level of 0.05% (two glasses of wine). This is the legal drink driving limit in the UK.

    Research also shows that sleep-deprived individuals often have difficulty in responding to rapidly changing situations and making rational judgements. In real life situations, the consequences are grave and lack of sleep is said to have been be a contributory factor to a number of international disasters such as Exxon Valdez, Chernobyl, Three Mile Island and the Challenger shuttle explosion.

    Sleep deprivation not only has a major impact on cognitive functioning but also on emotional and physical health. Disorders such as sleep apnoea which result in excessive daytime sleepiness have been linked to stress and high blood pressure. Research has also suggested that sleep loss may increase the risk of obesity because chemicals and hormones that play a key role in controlling appetite and weight gain are released during sleep.

    What happens when we sleep?

    What happens every time we get a bit of shut eye? Sleep occurs in a recurring cycle of 90 to 110 minutes and is divided into two categories: non-REM (which is further split into four stages) and REM sleep.

    During the first stage of sleep, we're half awake and half asleep. Our muscle activity slows down and slight twitching may occur. This is a period of light sleep, meaning we can be awakened easily at this stage.

    Within ten minutes of light sleep, we enter stage two, which lasts around 20 minutes. The breathing pattern and heart rate start to slow down. This period accounts for the largest part of human sleep.

    Stages three and four: Deep Sleep

    During stage three, the brain begins to produce delta waves, a type of wave that is large (high amplitude) and slow (low frequency). Breathing and heart rate are at their lowest levels.

    Stage four is characterised by rhythmic breathing and limited muscle activity. If we are awakened during deep sleep we do not adjust immediately and often feel groggy and disoriented for several minutes after waking up. Some children experience bed-wetting, night terrors, or sleepwalking during this stage.

    The first rapid eye movement (REM) period usually begins about 70 to 90 minutes after we fall asleep. We have around three to five REM episodes a night.

    Although we are not conscious, the brain is very active - often more so than when we are awake. This is the period when most dreams occur. Our eyes dart around (hence the name), our breathing rate and blood pressure rise. However, our bodies are effectively paralysed, said to be nature's way of preventing us from acting out our dreams.

    After REM sleep, the whole cycle begins again.

    How much sleep is required?

    There is no set amount of time that everyone needs to sleep, since it varies from person to person. Results from the sleep profiler indicate that people like to sleep anywhere between 5 and 11 hours, with the average being 7.75 hours.

    Jim Horne from Loughborough University's Sleep Research Centre has a simple answer though: "The amount of sleep we require is what we need not to be sleepy in the daytime."

    Even animals require varied amounts of sleep:

    Species Average total sleep time per day
    Python 18 hrs
    Tiger 15.8 hrs
    Cat 12.1 hrs
    Chimpanzee 9.7 hrs
    Sheep 3.8 hrs
    African elephant 3.3 hrs
    Giraffe 1.9 hr

    The current world record for the longest period without sleep is 11 days, set by Randy Gardner in 1965. Four days into the research, he began hallucinating. This was followed by a delusion where he thought he was a famous footballer. Surprisingly, Randy was actually functioning quite well at the end of his research and he could still beat the scientist at pinball.

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    Strange! Humans Glow in Visible Light

    The human body literally glows, emitting a visible light in extremely small quantities at levels that rise and fall with the day, scientists now reveal.

    Past research has shown that the body emits visible light, 1,000 times less intense than the levels to which our naked eyes are sensitive. In fact, virtually all living creatures emit very weak light, which is thought to be a byproduct of biochemical reactions involving free radicals.

    (This visible light differs from the infrared radiation &mdash an invisible form of light &mdash that comes from body heat.)

    To learn more about this faint visible light, scientists in Japan employed extraordinarily sensitive cameras capable of detecting single photons. Five healthy male volunteers in their 20s were placed bare-chested in front of the cameras in complete darkness in light-tight rooms for 20 minutes every three hours from 10 a.m. to 10 p.m. for three days.

    The researchers found the body glow rose and fell over the day, with its lowest point at 10 a.m. and its peak at 4 p.m., dropping gradually after that. These findings suggest there is light emission linked to our body clocks, most likely due to how our metabolic rhythms fluctuate over the course of the day.

    Faces glowed more than the rest of the body. This might be because faces are more tanned than the rest of the body, since they get more exposure to sunlight &mdash the pigment behind skin color, melanin, has fluorescent components that could enhance the body's miniscule light production.

    Since this faint light is linked with the body's metabolism, this finding suggests cameras that can spot the weak emissions could help spot medical conditions, said researcher Hitoshi Okamura, a circadian biologist at Kyoto University in Japan.

    "If you can see the glimmer from the body's surface, you could see the whole body condition," said researcher Masaki Kobayashi, a biomedical photonics specialist at the Tohoku Institute of Technology in Sendai, Japan.

    The scientists detailed their findings online July 16 in the journal PLoS ONE.

    Is This Human Body Part a Muscle or a Bone?

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