The pineal gland, also called the pineal gland, gets its name from its pinecone-like shape. In humans, it weighs 100 mg, has a length of 6 mm and a diameter of 3 mm. Located in the heart of the brain, it plays a central role in regulating seasonal rhythms and biological rhythms: day/night, wake/sleep.
The pineal gland secretes the “night hormone”.
The pineal gland synthesizes melatonin from molecules captured directly in the blood. It releases this hormone into the bloodstream and thus tells the body that night is coming. As a result, it promotes falling asleep and maintaining sleep. Melatonin is a hormone whose secretion is usually circadian, ie. its synthesis follows a cycle of about 24 hours. It is released into the blood at the end of the day, with peak production between two and four in the morning. Then its presence in the body decreases until the next evening.
The activity of the pineal gland is controlled by the internal clock.
© 黄雨伞 – CC-3.0 / adaptation for Science and the future Horia Bahri
Buried deep in our brains, without access to light, the pineal gland receives information about the cycle of day and night thanks to an internal clock located in the hypothalamus of the brain. Clocks are “set on time” by various external factors, the most important of which is light. When the retina receives light, a nerve impulse communicates to the hypothalamus. This signal then follows a complex path through the spinal cord to the pineal gland. She is then told that it is daytime: melatonin secretion is suppressed. On the other hand, if the retina does not receive light, the internal clock signals this to the pineal gland: then the secretion of melatonin is stimulated. Evening exposure of the eyes to artificial light, if it is intense and prolonged, shifts the secretion of melatonin: wakefulness remains active and falling asleep becomes difficult.
Some sleep disorders can be treated with synthetic melatonin. However, medical advice is preferable.
The activity of the pineal gland changes throughout life.
Melatonin secretion steadily declines with age. It reaches its peak productivity in a person’s life between 3 and 6 years of age. Its role in the onset of puberty is still debated. In old age, it can completely disappear, which is associated with the formation of stones and a decrease in the volume of the pineal gland.
The pineal gland and the third eye
In humans, and in mammals in general, the pineal gland is not a light-sensitive organ and is not directly associated with a light-sensitive structure. This does not apply to other vertebrates. Some reptiles have a parietal eye in addition to the two eyes that provide vision. This “third eye”, as it is often called, has photoreceptor cells. However, if it perceives light intensity, this organ cannot analyze the images. In some vertebrates, notably the green anole, the American lizard, this primitive ocular structure initiates melatonin synthesis.
Several hypotheses have been put forward to explain this difference between mammals and reptiles. One postulates that these two classes of animals share a common photoreceptor organ. In the case of mammals, this organ would have evolved: on the one hand, these photoreceptor cells would have lost the ability to perceive light and would have evolved towards other functions; on the other hand, some parts of the organ would form the pineal gland. This evolutionary scheme still causes a lot of controversy.