Jeong Ki-young, President of the Korea Society for Sleep Research (Professor of Neurology, Seoul National University Hospital)
In the last article we saw how a person’s biological rhythm can carry out periodic activities on its own even without external light. If a person lives in a cave or underground bunker for a long time without sunlight, the person’s daily cycle usually becomes longer than 24 hours.
The experimental conditions are different for each study and the ratios vary from 24.5 to 27 hours, but as a result of a detailed experiment conducted by a research team from Harvard University, it was found that the average human circadian cycle is approximately 24.2 hours, regardless of age.
In the absence of light, we will be under the control of a biological clock that lasts longer than 24 hours, and each day will be at least 12 minutes longer than 24 hours, and we will go to bed 12 minutes later and wake up later each day. However, in daily life, it is not often that we notice a slowdown in our biological clock. Reset your body clock cycle by exposing yourself to light every morning.Because it becomes one.
The length and amplitude of the circadian cycle of the suprachiasmatic nucleus, an autonomously oscillating central biological clock, are influenced by various internal and external factors. Among external stimuli, light is the most powerful regulator of the biological clock. When you wake up in the morning and expose yourself to sunlight, your body clock restarts the day from that point.Do. In other words, morning sunlight serves as a signal signaling the start of the day.
In this way, the process by which our body’s biological clock adapts its time to sunlight is called “resetting” or “synchronizing” the biological clock. The reason we wake up later in winter is because sunrise is delayed, we are exposed to sunlight later and our body clock starts later the day.
On the other hand When it gets dark at night or you are exposed to a very low level of light, your body clock recognizes that night has come and switches to sleep mode, which secretes melatonin and lowers your body temperature.
However, if you expose yourself to too bright light before bed, your body clock still recognizes that the day continues and the body clock cycle becomes longer, causing you to fall asleep later and wake up naturally later. Sunlight induces synchronization not only with the daily cycle but also with seasonal changes. This helps increase the survival of individuals by allowing them to predict and adapt to seasonal changes.
How does the suprachiasmatic nucleus, located in the center of the brain, detect light and be affected by the biological clock cycle? Let’s take a closer look at the process by which the biological clock detects light and adapts its clock to the external clock.
The retina of the eye contains cells with photoreceptors that detect light, dark, and color. The rods distinguish between light and dark in dim light and are responsible for night vision. Cone cells recognize colors in bright light and provide high-resolution visual information. The reason why brightness and color can be detected is because there is a light-sensitive protein called opsin in visual cells. Rod cells have one type of opsin and cone cells have three types of opsin. Furthermore, the wavelength of the detected light is different depending on the type of opsin and as a result it is possible to distinguish light and dark colors.
The detected visual information is transmitted to the visual center in the occipital lobe of the brain, allowing the identification of objects. People who have completely lost their sight are unable to perceive light. Since your body clock doesn’t reset due to light, you can expect to sleep a little later and wake up later each day.
However, it is interesting to note that most visually impaired people, like the general public, live on a 24-hour cycle because their body clocks are reset by sunlight. The same was true in animal experiments. It has been confirmed that mice genetically lacking rods and cones have their biological clocks adapted to light and dark, just like mice with normal retinas. Although it has not seen the light of day in the scientific community, the reason why the biological clock works correctly has not yet been discovered for some time.
In the 1980s, scientists already knew that frog skin can change color in response to light. This phenomenon is particularly sensitive to blue light, which plays an important role in helping frogs adapt to their environment, hide from predators, and attract mates.
By studying the light-responsive mechanism of frog skin, it was discovered that some skin cells called melanophores contain a protein similar to opsin, a light-sensitive protein. The researchers called this opsin-like protein melanopsin.
Later, in 2000, melanopsin was identified in human retinal ganglion cells. Melanopsin was confirmed to be a third photoreceptor different from existing opsins as it is located in a different layer of the retina and has different genes, unlike the rods and cones previously responsible for vision.
As mentioned above, circadian rhythm synchronization is maintained in mice from which rods and cones have been removed. Melanopsin plays a role in synchronizing the circadian rhythmI thought about doing it.
Melanopsin-containing ganglion cells were soon confirmed to be connected to the suprachiasmatic nucleus, the brain’s central biological clock, and were found to respond sensitively to wavelengths of blue light. Melanopsin is present in very small quantities in only 1% of all retinal ganglion cells, and these cells have been found to be sensitive to light, hence the rather long name intrinsic light-sensitive retinal ganglion cells (ipRGCs). Unlike rods and cones, ipRGCs are more sensitive to blue light wavelengths around 480 nm, like melanophore cells found in frog skin.
Retinal ganglion cells (ipRGCs), which contain melanopsin, play a central role in providing light stimulation to various parts of the brain and performing the nonvisual functions of light. ipRGCs regulate physiological processes according to the day and night cycle by transmitting information about external light to the suprachiasmatic nucleus, the brain’s biological clock.Do.
This process influences sleep patterns, hormone secretion and other body functions. Additionally, ipRGCs mediate the pupillary reflex, which causes the pupil to constrict in response to bright light, which helps regulate the amount of light entering the eye and prevents retinal damage from excessive light. ipRGCs can influence mood and behavior, and the light information they process can affect mood-related neurotransmitters such as serotonin levels, making them associated with seasonal affective disorder (SAD).
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