by Carol A Westbrook
On the first day of creation, God said. “Let there be light,” according to the Bible. In most major religions, God creates day and night, the sun, or light itself on the first day. To the ancients, the sun was God Himself–the Egyptians had Ra, and the Aztecs, Huītzilōpōchtli.There is something divine about light. Even a non-religious person feels a sense of something special when in bright sunlight.
Sunlight! We crave it. We open the drapes, we spend time in the sun, we gaze at the sunset, we build stone monuments to track it. In the dark days of winter we brighten our homes with candles and holiday lights. Winter religious holidays like Christmas and Chanukah emphasize lights and candles to brighten the darkness, while other holidays come during spring, when the days get longer. Many of us feel the need to travel south in the dead of winter, to get a few days of bright light and longer days.
We don’t need to invoke religion to explain our craving for light, we can look to biology. Light–specifically daylight–is a human need, almost as critical as food, air and water. We need the periodicity of daylight to control our bodily processes, in particular those which occur in a diurnal cycle, such as sleeping, waking, meals, drowsy times, body temperature variations, and fertility.
Hormones control this periodicity. These include cortisol, which controls blood sugar and metabolism, and melatonin, a sleep regulator.
Cortisol levels are high on awakening and decrease over the course of the day, as less energy is needed and food intake goes down. Melatonin increases at bedtimes and during sleep, but is low during the day. There are many other hormones that follow an internal clock, known as our circadian clock.
Circadian rhythms have a molecular basis that is built into our genes. The discovery of these was so important that the 2017 Nobel Prize was awarded to the two scientists who identified these genes, Ron Konopka and Seymour Benzer.
Most living creatures have circadian clocks. Their bodily rhythms provide an adaptive advantage, providing the biologic urges for essential activities at the appropriate time of day. Good timing helps to avoid predators, to find food when it’s easiest to spot, to prepare for mealtimes, and to sleep regularly. In social animals, diurnal clocks are especially important because they coordinate group behaviors and even mating, so it’s important to synchronize with the rest of the groups. Our internal circadian rhythm repeats approximately–but not exactly–every 24 hours, but some people’s clocks are much longer or shorter. That’s why we need to re-calibrate our clocks to coordinate with the solar cycles, matching other people in our community.
The way we re-calibrate our clocks is by exposure to sunlight. Training our diurnal clocks to the existing daily cycle is called “entrainment.” This is done directly into our brain through our vision. Scientists have found that artificial bright blue light of wavelength 480 nanometer is also effective, whereas red light is weak. That’s why nocturnal animals in zoos can be observed as awake and fully active under red light. Colorblind people can entrain their clocks, but it’s harder for blind people who must rely on perception of very small amounts of light or on external cues to set their diurnal rhythms. Many blind people have sleep disturbances.
I believe that this drive to keep our clocks in time is the reason that we are driven to glorify sunlight. Re-setting our clocks is such an essential part of our nature that we don’t even notice it. That is, we don’t notice it until our clocks are out of synchrony. If your diurnal rhythm is off by even a few hours you can tell. Ever been jetlagged? Or abruptly changed to night shift without preparation? You will have insomnia or sleep disturbances. You will be hungry at the wrong time–but not at mealtimes. You feel generally miserable until you entrain your circadian clock to local time.
Human biology needs sunlight for other reasons. We need it to make Vitamin D, necessary to absorb calcium. And approximately 10 percent of humans suffer from seasonal affective disorder, or SAD, when the shortening days of autumn trigger depression, which is sometimes so severe that it leads to suicide. Many SAD people travel south for the winter, others use light treatment, and still others just do the best they can. But contrary to popular belief, shorter days do NOT trigger hibernation and make us eat more in the winter. Hibernation is triggered by the falling temperatures, and humans do not have hibernation genes.
There is another reason that we worship the sun–it is the source of all of our energy and food. As such, it is the source of life itself. Our food comes directly or indirectly from photosynthesis, which produces plants for our diet and that of our animal sources. And the sun provides the energy that all living creatures need to keep warm and to cook. The sun is the source of all of our fossil fuels, the stored products of photosynthesis from eons past. Even alternative power sources such as wind and hydroelectric power result from the wind currents propelled by the sun’s heat. There would be no wind or weather if then sun were dark.
There cannot be life without sunlight, either on earth or elsewhere in the solar system. And probably not elsewhere in the universe. Or can there?
Surprisingly, scientists have discovered two forms of life that live without any source of light for photosynthesis. The first of these is a bacterium that lives near geothermal vents deep in the ocean. They create their life’s fuel, sugar, from water and inorganic chemicals (hydrogen and hydrogen sulfide from rocks and sea water), using heat energy instead of sunlight; this is their version of photosynthesis without light.
Ever more strange are small bacteria called D. audaxiator. They were discovered in 2006 within rock found within South Africa’s deepest goldmine. This bacterium lives completely enclosed within a cage of hard, dense volcanic rock, sitting in a pool of hot (140°), sulfurous, ancient water, 3 billion years old! This creature manufactures its organic molecules from water, inorganic carbon and ammonia from surrounding rocks and fluid. Its energy source is heat generated from decay of radioactive uranium. This organism’s version of “photo”synthesis could be called “radio”synthesis!
The discovery of these remarkable living creatures, which have no need for the sun, leads us to realize that the variety of life in the universe is much broader than we originally thought. Furthermore, the climate and conditions under which life might arise on other plants can stretch our imagination. What kinds of life will we find in the outer reaches of the solar system, or deep in the oceans of Saturn’s moon Europa, where little or no light penetrates?
Food for thought. Now I have to run. I have a plane to catch, going south for a winter break…