Summary 4. Circadian Rhythm Is Controlled By Light – Nature Is A Great Teacher.

Since the sun and the earth were born about 4.5 billion years ago and the first biological organism showed up on the earth between 3.7 and 4.3 billion years ago, all the living organisms on the earth have evolved under the influence of sunlight (Figure 1). What is interesting is that the innate biological cycle of biological organisms of 24.3 hours per day is adjusted to 24 hours by sensing the cycle of sunlight.

Figure 1. All the biological organisms on the earth have evolved to survive under or to make the best use of the sunlight.

Our body fine-tunes our body clock by perceiving natural light on photoreceptors mainly in our eyes. Our body clock well synchronized with the daily sunlight cycle leads to secretion of melatonin from pineal glands during the daytime helping us awake and inhibits it at night for sleep of high quality and also slows down proliferation of cancer cells (Figure 2).

Figure 2. Humans control secretion of melatonin from pineal glands by perceiving natural light mainly through eyes. Lower level of melatonin during the daytime inhibits melatonin helping us awake, whereas its high level at night leads to sleep of high quality and also delays growth of cancer cells (Photo credit : Journal of Clinical Sleep Medicine).

More than 90% of plants on this planet have the Fibonacci sequence in their structure. The Golden Ratio of approximately 1:1.6 (more precisely, 1.61803398875…) coming from the nature is perceived as the most beautiful and the most comfortable to human eyes (Figure 3A). Phyllotaxis or the arrangement of leaves is also ruled by the Golden Ratio – every new leaf sprouts at every 137.5 degrees that is divided 360 degrees by the Golden Ratio. In that way, plants get the maximum sunlight and optimize photosynthesis at any given moment (Figure 3B).

Figure 3. (A) Fibonacci Sequence and the Golden Ratio of approximately 1:1.6. Finabocci Sequence is a series of numbers, where any number in the sequence becomes the sum of two previous numbers except the first two. The bigger number divided by the smaller of any two consecutive numbers on this Sequence also generates approximately 1.6 (as the numbers get bigger, the resulting number approaches 1.61803398875…), and the ratio of 1:1.6 is called the Golden Ratio, (B) Phyllotaxis or the arrangement of leaves on plants is also ruled by the Golden Ratio. 360 degrees divided by the Golden Ratio result in 137.5 and 222.5 degrees. In the figure, every next leaf comes out at 137.5 degrees. In this way, this plant can get the most sunlight and optimize photosynthesis at any given time (Modified from

The Nobel Prize of Physiology or Medicine was awarded to three scientists who made critical contribution to understanding of the molecular mechanism of the circadian clock of fruit flies, Drosophila (Figure 4). The level of PER protein produced from Period gene in Drosophila oscillates at approximately 24-hour cycle, reaching the maximum in the early evening and the minimum in the morning, playing the central role in orchestrating the molecular mechanism of circadian rhythm.

Figure 4. The Nobel Prize Winners in Physiology or Medicine in 2017 “for their discoveries of molecular mechanisms controlling the circadian rhythm”. From left to right, Drs. Jeffrey C. Hall, Michael Rosbash and Michael W. Young (

Per gene shows high level of homology throughout many living organisms from human and mouse to even dinosaurs who lived about 700 million years ago. It reflects the importance of circadian rhythm well synchronized to natural cycle of day and night and may be the reason why it is said “Nature is a great teacher”.

Biologist A-Young Lee Ph.D
Biologist Jin-won Kim Ph.D
Biologist Jae-ho Lee Ph.D
Biologist Sang-wook Chung Ph.D
Physicist Sung-soo Kim Ph.D
Electronic and Information Engineering Sooyoung Moon Ph.D
Physics & MBA Chung H. Lee Adviser

* Next week’s topic: How circadian rhythm works in biological organisms

You can find "Interesting Story about Light Science” series on the link below.

Seoul Semiconductor /
97-11, Sandan-ro 163beon-gil, Danwon-gu, Ansan-si, Gyeonggi-do, Republic of Korea