Working at sunrise and resting at sunset

, these life experiences accumulated by our ancestors have been naturally integrated into our bodies. Like many creatures, our bodies have evolved a circadian rhythm that coincides with the alternation of day and night, or simply, we call it the biological clock.

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We usually think that rhythm is to go to bed on time, but in fact, in addition to sleep, the body temperature, cognitive function, neuroendocrine and so on all have rhythm. Make the physiological activity and behavior rhythm of the body consistent with the periodic changes of the external environment, which is also called entrainment. The mechanism that regulates the rhythm of the human body is very complex, and it all starts with light, which is also a factor in the periodic changes in our perceived environment.

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In ancient times, or even not long ago, when night came, the human world was mostly dark (or at least dark). In recent years, with the increasing popularity of artificial light sources, especially electronic products, the impact of light on human rhythm has been paid more and more attention, of which blue light is the most frequently mentioned. The wavelength range of human visible light is about 400~760nm, in which the shorter wavelength (about 450~485nm) of blue light is often considered to have a negative effect on the sleep rhythm of the human body.

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However, how do these electromagnetic waves that bring us light make our bodies produce rhythms, and how do they seriously affect our rhythms?

Rhythm controller

You might say, of course, you have to look with your eyes. This answer is half right-with the eyes, but not through the traditional sense of "seeing".

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Of course, the important task of photosensitivity is inseparable from our most important eyes, especially the retina at the back of the eyeball. Cone cells and rod cells are known as photosensitive cells, the former can sense color and brighter light, while the latter play a role in dark environments. In addition, there is a third kind of photosensitive cells in the retina-intrinsic light-sensitive retinal ganglion cells (ipRGCs).

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Unlike the first two kinds of cells, ipRGCs are non-imaging cells, they are not responsible for "seeing" the colorful world, they can only feel the intensity of light. IpRGCs transmits signals to the suprachiasmatic nucleus (suprachiasmatic nucleus) in the hypothalamus through the retinal hypothalamic nerve bundle (RHT).

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The suprachiasmatic nucleus is located above the optic chiasma (an X-shaped structure formed from the optic nerve intersection of both eyes) and on both sides of the third ventricle, each containing about 10,000 neurons. Although this tiny structure is inconspicuous compared to the 14 billion neurons in the entire brain, it is the command center of our biological rhythm.

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There is a set of molecular mechanism of feedback loop in the suprachiasmatic nucleus, which can carry out rhythmic activities spontaneously and continuously, which is equivalent to a natural "clock". Moreover, the suprachiasmatic nucleus coordinates the rhythms that exist in these regions by projecting to the nerves that control arousal, sleep, neuroendocrine, autonomic nervous system, and so on, so it acts as a pacemaker of human body rhythm, helping our bodies obtain 24-hour rhythms from sunrise and sunset (rhythm cycles vary slightly from person to person).

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Blu-ray became the culprit.

When it comes to the dangers of blue light, you have to introduce melatonin (melatonin), another big star who often appears in the same frame as sleep problems. Melatonin is a hormone secreted by the pineal gland in the brain that regulates circadian rhythms. Light inhibits melatonin secretion, which is regulated by the suprachiasmatic nucleus. In the natural state, our melatonin secretion begins to increase shortly after sunset, peaking at 2: 00-4: 00 in the morning, and then gradually decreasing, showing such periodic changes every day. The periodic changes of melatonin content in the body are often used as an index to reflect the circadian rhythm.

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Since more than 20 years ago, many studies have found that blue light can more effectively inhibit melatonin secretion and change the rhythm of melatonin secretion than longer wavelength light and mixed white light. As a result, people began to point the finger at blue light.

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It is worth mentioning that at that time, scientists also believed that only cone cells and rod cells in the human body had photosensitive function. However, they found something suspicious in the study of melatonin. Rod cells are most sensitive to light with wavelength about 500nm, while cones are most sensitive to light with wavelengths about 430nm, 530nm and 560nm, respectively. Although the results of these studies are slightly different, it is generally found that blue light at wavelength 440~480nm has the most significant effect on rhythm, and such action spectra (action spectrum, the curve of the efficiency of light-induced physiological or chemical reactions varying with the wavelength of light) are not consistent with the characteristics of the two classical photosensors.

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So scientists speculated that there were other photosensitive cells or proteins in the human body, and subsequent findings confirmed this. It turns out that there is a unique optomelanin (melanopsin) in ipRGCs, which is most sensitive to blue light with a wavelength of about 480nm (also thought to be 460nm). In this way, everything makes sense, and Blu-ray has naturally become the chief culprit for disrupting our biological clock.

It's not just blue light.

However, the human body is a very complex and sophisticated system, and the rhythm regulation mechanism is no exception. Some studies have found that longer wavelengths of green and red light can also affect people's circadian rhythm, while some studies have found that the strongest inhibition of melatonin secretion is not blue light, but shorter wavelength purple light. Moreover, the accuracy of many studies is not high enough to determine that it is the visual melanin that affects the body's rhythm by sensing blue light. As a result, there has been controversy in the scientific community about letting Blu-ray take the blame.

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Scientists believe that in addition to visual melanin, other components of the photosensitive system may also be involved in the regulation of circadian rhythms. For example, there is evidence that rods and cones communicate with ipRGCs, and ipRGCs may also receive signals from these two cells and integrate a variety of light signals to respond to various wavelengths of visible light.

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A recent study supports this view. The researchers irradiated the eyes of subjects with purple (405nm), blue (470nm), green (515nm) and orange (590nm) light pulses and found that except purple light had no effect, the other three kinds of light significantly inhibited the neural activity of the suprachiasmatic nucleus, and there was no significant difference between the effects.

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In addition to the spectral characteristics, the intensity and duration of light also affect the effect of light, and their effect on rhythm has a dose effect, so we should not only care about the light of a particular color. A 2015 study found that people who used iPad fell asleep longer, secreted less melatonin and delayed their circadian rhythm than those who read traditional paper books. The authors of the study believe that this is because the light emitted by the iPad screen is mainly short-wavelength light (the spectral peak is at the blue light of the 452nm), while the ambient light when reading a paper book is white (the spectral peak is at the orange light of the 612nm).

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However, in this popular study, the color of light is not the only variable. In fact, the subjects were in a dim environment when reading paper books, with the light intensity more than an order of magnitude lower than that of i