Deep inside your brain sits a very small endocrine gland called the pineal gland. This tiny gland secretes the hormone melatonin, and in this lesson, you will learn about the role melatonin plays in regulating sleep and wake cycles.
The pineal gland is the size of a pea and is located between the two hemispheres of the brain.
Compared to other organs in your body, the endocrine organs are small and quite modest in appearance. In fact, if you were to gather up all of the endocrine organs in your body and put them on a scale, they would weigh only a few ounces. However, their small size is not a measure of their power.
Hormones from your endocrine organs control everything from how you grow to how well you sleep. In this lesson, we will focus on one of the tiniest endocrine glands, known as the pineal gland, and learn about the hormone it produces called melatonin. The pineal gland is a tiny endocrine gland found in the brain. It's no bigger than a pea and can be found situated between the right and left hemispheres of your brain. It gets its name from the Latin word 'pinea,' which means 'pine cone,' because when we look at the gland, we see that it is shaped like a cone.
While the full function of the pineal gland is still somewhat mysterious, we do know that it secretes the hormone melatonin. Melatonin is a hormone from the pineal gland that helps regulate biological rhythms such as sleep and wake cycles. In other words, it's melatonin that directs your internal clock - the one that allows you to wake up about the same time every morning without an alarm clock.
Melatonin levels peak at night and promote restful sleep.
Melatonin is secreted in different amounts during the night and day. Melatonin secretions peak during the nighttime, and this makes you feel drowsy. While it does not necessarily induce sleep, it can promote sleep and allow a good night's rest. In fact, we see melatonin marked as a supplement to help people who deal with insomnia. Because melatonin secretions peak during puberty and then continue to drop throughout life, it is thought that a lack of melatonin production is why so many elderly people report trouble sleeping.
You may be familiar with the term circadian rhythm. This is the daily cycle of biological activity based on a 24-hour period. This cycle can be disrupted by changes in your daily schedule such as working the night shift. This is because the production of melatonin is triggered by darkness and inhibited by light exposure to the eyes. When a worker is on the job during the dark hours and then closes the blinds and sleeps during the daylight hours, secretions of melatonin get thrown out of whack.
When the retina is exposed to light, melatonin production shuts off.
Here's how it works. When the retina of your eye is exposed to light, a message is sent to your hypothalamus. Fibers from the hypothalamus then transmit this message down the spinal cord to clusters of sympathetic nerve cell bodies that then ascend back up to the pineal gland. When this message reaches the pineal gland, melatonin production is turned off. So, we see that the pineal gland receives signals from the sympathetic nervous system. This is a process only seen in a few endocrine organs, such as the adrenal medulla.
Jet lag, which is defined as fatigue or other physical effects caused by high-speed travel across time zones, is another way your body's circadian rhythm can be disrupted. This disruption is most noticeable when flying across several time zones. While the body's clock can adjust to new environments gradually over time without any noticeable symptoms, this rapid change can disrupt the body's sleep and wake cycles. This makes it difficult to sleep through the night or stay awake during the day and leaves you feeling fatigued and irritable all thanks to the disruption of the normal melatonin secretions from the pineal gland.
Seasonal Affective Disorder
Light therapy can benefit a person with seasonal affective disorder.
We see that melatonin plays a role in regulating daily biological cycles, but changes in melatonin production have also been linked to seasonal mood rhythms. Seasonal Affective Disorder, or SAD, is a kind of depression associated with the winter months that is thought to be caused by shorter periods of daylight. During the winter months, days get shorter, especially in regions far away from the equator. This lack of sunlight results in higher-than-normal productions of melatonin, which leads to sleepiness and low energy levels. A person with SAD may be treated with light therapy, which is the use of very bright lights for a couple of hours a day. This added light exposure acts like artificial sunlight and slows the production of melatonin.
Let's review. The pineal gland is a tiny endocrine gland found in the brain. It produces and secretes the hormone melatonin, which is a hormone that helps regulate biological rhythms such as sleep and wake cycles. The secretion of melatonin is inhibited by light and triggered by darkness. When it is produced, it makes you feel drowsy, and therefore we see it used as a dietary supplement for such conditions as insomnia.
Your circadian rhythm, which is the daily cycle of biological activity based on a 24-hour period, is linked to melatonin production. When light hits the retina of your eye, a message is sent through your hypothalamus to the sympathetic nervous system and then to your pineal gland telling the gland to inhibit the release of melatonin. This allows you to wake up and get on with your day. However, this normal circadian rhythm can be thrown off by factors including jet lag and working the night shift.
Seasonal Affective Disorder, or SAD, is a kind of depression associated with the winter months, which is thought to be caused by shorter periods of daylight. The lack of sunlight during the winter months leads to sleepiness and low energy levels due to higher production of melatonin.
After this video lesson, you'll have the ability to:
- Identify the location and function of the pineal gland
- Explain how melatonin helps regulate sleep and wake cycles
- Describe what the circadian rhythm is and explain what happens when that rhythm is thrown off
- Summarize the causes of seasonal affective disorder and how to treat it