In order to make the best perfume, people pick roses from midnight to two o’clock in the morning before the morning dew dries up; because roses have the best fragrance at dawn when is dark and cold. Then why are roses more fragrant at dawn? How do they know what time it is and emit fragrance? It is not only roses that do so. Around noon every day, your stomach growls as if it knew what time it is. You have lunch, come back, and start working again. But then around 3 in the afternoon, you feel drowsy as if it was already planned to. When the sun rises, you wake up, and when the night comes, you feel sleepy. As if there were a clock in the body, numerous animals and plants, including humans, repeat many things in the cycle of a day, a month, and a year.
When you go to a distant country for a trip by plane, you struggle with jet lag for several days. You often doze off when the sun is high up in the sky; your body wants to sleep in the middle of the day as if it were night. Also, when the night comes, you just can’t fall asleep. It is because your body tries to keep the flow of 24 hours of the country you stayed before, regardless of the time of the current location. This circadian rhythm is too accurate to simply call it a result of a habit. Then how is it made?
The circadian rhythm of living organisms was first discovered in the 4th century B.C. Androstenes, one of the generals that led Alexander the Great’s expedition, recorded that the tamarind tree’s leaves were horizontal during the day and vertical at night. After some time, similar periodicity in plants was discovered. In 1729, Jean Jacques d’Ortous de Mairan, a French astronomer, spotted something strange while observing mimosa. The mimosa’s leaves fold up and look withered when touched. However, de Mairan saw the mimosa’s leaves unfold during the daytime and fold up at night even without external stimulus. At first, he thought it was due to the light. However, the mimosa’s leaves unfolded during the daytime and folded at night even in a completely dark room.
Not only plants but also human body shows a similar phenomenon. In 1960, the Max Planck Society of Germany had an experiment to see if the circadian rhythm in humans would stay constant even in the basement with no windows. As a result, they observed that most people went to bed and woke up in the cycle of approximately 25 hours. It means there is a circadian clock in the human body and that it keeps certain rhythms. It also shows that the fundamental cause of the circadian rhythm is not the external factors such as light, but the internal factors of living organisms.
The first observation of the existence of periodicity in animals and plants dates back to a very long time ago. However, the answer to the fundamental question started to come out as the research on the drosophila’s genes began. Scientists discovered that what operated the circadian clock was genes. They discovered the genes that operate the circadian clock from the fact that the drosophila is developed from larva into imago only in the early morning. They discovered genes such as clock, period, and timeless, which control the circadian rhythms in drosophilae. The circadian clock is controlled by the interaction of the proteins made by these genes. An analog clock, which has a complicated inner structure, has several saw-toothed wheels that move in interaction and operate the hands of the clock. In the same way, the circadian clock of our body operates systematically as the several proteins made by the genes interact. In other words, each and every cell has a complicated, exquisite clock.
The average of a human’s daily cycle is 24.3 hours, not perfectly 24 hours. It varies, depending on people, so there are “morning people” whose cycle is close to 23 hours, and there are “night owls” whose cycle is close to 25 hours. The cells in our bodies show slightly different periodicities from each other.
Then how does a living organism not go against the periodicity of 24 hours? It is because they have the central circadian clock which senses the light and controls all the circadian clocks inside the body. The part that functions as the central circadian clock is suprachiasmatic nucleus [SCN] which is the size of a rice grain, located in the hypothalamus of the brain. The SCN which consists of 20,000 nerve cells and is connected with the optic nerves of the two eyes, senses the changes of the sunlight and sends signals to various parts of the body to reset the circadian rhythms to be 24 hours. The sunlight itself is not the cause of the circadian rhythms, but it provides the standards to set our circadian clock to be at certain hours every day. So, if a problem occurs in the SCN by a brain tumor, all the rhythms of a daily life break, which causes waking up and sleeping at random times. The fact that the central circadian clock is in the SCN has been discovered, but its accurate mechanism and the mystery of the synchronization of cells have not been solved yet.
The SCN which contains many mysteries secretes hormones and controls sleep. When the SCN senses a change in light and that it has become dark, a signal is conveyed to the pineal gland and the pineal gland secretes melatonin which is a hormone that helps you get deep sleep. By the signal coming from the SCN, melatonin is secreted when it’s dark, and is not secreted when it’s bright in the daytime. The secretion of melatonin helps us fall into deep, restful sleep. However, the number of people suffering from sleep disorder is increasing as we’ve entered the digital age. The reason people cannot have sound sleep is because the secretion of melatonin decreases as they are exposed to the bright light such as the cell phone or TV at night; the SCN mistakenly thinks it is a daytime.
Some people suffer from more extreme sleep disorder. It’s because their circadian clock is broken. People who suffer Delayed Sleep Phase Syndrome [DSPS] normally sleep from four in the morning until noon. It is almost impossible to do anything in the morning like ordinary people. Whereas, people who suffer Advanced Sleep Phase Disorder [ASPD] normally fall asleep around 7:30 in the evening and wake up around 4:30 in the morning; they are basically extreme morning people. According to a recent research, people who suffer ASPD show such signs because their genes that control the circadian clock have mutated.
We check time tens of times a day to see exactly what time it is and spend a busy day. However, our body doesn’t need to check the time to know when to sleep and when to eat. The color density change of plants, the fragrance emission of flowers, and the movements of leaves are all done in the right time. Insects too live according to certain rhythms from the moment they leave the pupal state until they live as imagoes. All things know the flow of time by instinct and live according to that flow. When it comes to humans, we do not only show rhythms for going to sleep and waking up, but also show the rhythms where the body temperature or the amount of hormone secretion changes in the periodicity of a day. Amazingly, the most accurate clock that suits the human body is not the watches around our wrists, but the tens of trillions of circadian clocks in our bodies. Thanks to the clock inside the body, we can feel the flow of time and live according to the time. All living organisms on the earth are living according to the flow of time which has already been input in them.
- Reference
- Stefan Klein, The Secret Pulse of Time: Making Sense of Life’s Scarcest Commodity, Da Capo Lifelong Books, 2009
- Sato Tomio, Magical Sleep: Dreams Come True while Sleeping (in Japanese, 魔法の快眠術: 眠りながら夢がかなう), 東洋経済新報社, 2005
- Jeong Jae-seung and 4 others, Things That We Barely Know about Humans and the Universe (in Korean, 인간과 우주에 대해 아주 조금밖에 모르는 것들), 낮은산, 2012
- YTN Science Documentary S, Alarm Clock in Body, Circadian Clock (in Korean, 몸 안의 자명종, 생체시계), 2013