How do animals, birds and plants foresee the seasons? They get to know the arrival of summer by sensing the lengthening days in spring; similarly winter, from the shortening days in autumn. If they sense it through the temperature fluctuations alone, then how do they adapt with an unseasonable warm or dry weather? Light crucially regulates the ‘biological clock’ in day-to-day life, and even in longer cycles as in hibernating polar animals. Human beings are also influenced by day light; morning light wakes and energises us, while the dimmer night light signals us to sleep and rest. Sunlight resets the biological clock daily, keeping it in sync with the 24-hour day. Imagine yourself in an underground shelter with constant artificial light. You would continue to follow a roughly 24-hour sleep-wake cycle, but the pattern would slowly get out of step with the actual day and night. The familiar ‘jet lag’ experienced by air travellers arises from passing through different time zones – it is our biological clock disagreeing with the watch on the wrist. Of course, the internal clock would eventually reset itself in time.

In fact, light has a significant bio-stimulating effect on humans as well as on animals and plants. ‘Ensuring that we receive good quantity of light, at the right intensity, at the right time of day, helps our conscious level, emotions, productivity, sleep patterns and many of our physiological aspects,' suggests recent studies. Exposure to adequate light levels act as cues for our internal biological clock that controls the ‘circadian rhythm’ which are the physical, mental and behavioural changes that follow a roughly 24-hour cycle, responding predominantly to day and night. The Latin word “circadian” means ‘about a day’; the circadian rhythm, our internal biological clock ticks and cues throughout your day-night cycle. ­ These rhythms are produced by many natural, biological factors linked with our body, and are influenced by many external factors, light being the main cue, which influence the ‘on’ and ‘off’ mechanisms of clock genes that control an organism's internal clocks. These genes are sets of instructions that code for clock proteins found in most living beings. The daily fluctuations in these protein levels regulate physiological characteristics in humans including sleep/wake cycles, hormone release, metabolism, body temperature, oxygen intake, cardiac activity and many more. A "master circadian clock" that is believed to synchronize all the "local" clocks residing in different organs and tissues throughout the body, is found in the region called suprachiasmatic nuclei (SCN) in the hypothalamus, an area of the brain just above where the optic nerves from the eyes cross. SCN is made up of tiny clusters of thousands of nerve cells that ‘tell time’ based on the external signals such as light and darkness.

The existence of some mechanism had been hypothesized, which detects transmission of day-night cycle from the eye to the ‘master clock’ in the brain. It was earlier thought that the resetting of the circadian clock was caused by the action of rhodopsin, a light-detecting protein in the rods and cones in the eye. Obviously, this failed to explain the internal clock resetting phenomenon in blind people (lacking rods and cones), who, nevertheless, are able to regulate their biological rhythms. Recent studies, however, identified a separate light detection system in the eye capable of gauging overall brightness to help reset the biological clock. This theory focuses on melanopsin, a protein which is sensitive to blue light and it resides in a small fraction of the eye's light-sensitive "retinal ganglion cells", which carry the information to the brain’s master clock.

Understanding the exact interplay between sunlight and biological clocks can help researchers decide the treatment measures for many health problems like, sleep disorders, jet lag, obesity, diabetes, depression, and even in treating cancer as many of the known clock genes are known to coordinate normal functions such as, cell growth and cell suicide. Light therapy is considered effective in treating certain types of depression that appears during the shorter days of winter. Some patients are exposed to the bright morning light under the assumption that light therapy can be made more effective by synchronizing with the patient's circadian clock. Vitamin D deficiency is another major worldwide challenge among the elderly, and not uncommon among children and adults. Sunlight is the major source of Vitamin D whose deficiency may lead to bone-softening diseases. Particularly, people in Polar Regions suffer from want of enough sunlight; doctors often advise them to take vitamin D supplement. But, as with any other radiation, risks are involved with sun radiation also. Overexposure to sun, especially between 11 am to 4 pm, can be avoided as it may leads to potential health hazards due to the presence of harmful ultraviolet (UV) rays. Being exposed to too much UV rays may cause serious skin cancers like melanoma; premature aging; skin damages like tans, melanin pigmentations; eye damages; and wakening of the immune system. A good understanding of such risk factors may help us to take reasonable precautions so that we can enjoy the sun while minimizing the chances of sun-related health problems.

Leaving aside the scientific aspects, if we look at this in a spiritual plane, we can see many references to light in ancient texts and religious scriptures. Bible says that God is light. Brahman (The unchanging reality) is light, says the Maitri Upanishad. Now, science agrees more and more with the fact that, besides the atoms and molecules that make up our bodies, we are beings of light as well. So, how can we ignore the importance of light in our life? Yes! Light is ‘life’, and so, precious too! In this International Year of Light, let us remind ourselves about the importance of light to health. Do let us “Enlighten” ourselves and stay healthy!

[ An article by me published in Executive Knowledge Lines(EKL)]