Summary 5. The Action and Influence of Circadian Rhythm (Morning Light and Evening Light)

As discussed in the previous summary, our body micro-tunes circadian clock to the 24-hour cycle through perception of sunlight by photoreceptors located on eyeballs and other parts. Natural light has different light spectra in the morning, afternoon and evening (Figure 1), and our body recognizes continuously changing sun spectrum and knows roughly the time during the day time.

Figure 1. Spectra of sunlight in the morning, afternoon and evening (Photo credit: Graph, Proc. Royal Soc. B.; Sunrise and Sunset,; The sun in the middle of the day,

Light signal is delivered from eyes to brain leading to changes in the hormones such as melatonin and plays an important role in maintaining circadian rhythm, making us awake during the day time and sleeping during the night time. The spectrum of sunlight in the evening is specifically lower in green and blue regions of comparatively short wavelenghths. Conventional white LEDs emit light with high ratio of blue-green region compared to sunlight in the evening, which prevents us from falling asleep quickly and having comfortable night sleep, whereas SunLike LEDs shed light close to natural sunlight helping us maintaining proper circadian rhythm and are expected to make great contribution to our healthy and comfortable sleeping (Figure 2).

Figure 2. Spectra of light from conventional white LEDs (left), natural sunlight (middle) and SunLike of Seoul Semiconductor (right).

The importance of circadian rhythm is recognized by Nobel Price and its effects on our health are listed in the table below (Table 1).

Circadian rhythm well maintained under the natural light plays a central and critical role in our health. Living under the natural light of spectrum that changes in the morning, afternoon and evening should be the best natural remedy for people living a majority of time indoor in modern times.

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: photon and animals/plants

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