Pineal Gland, Functions, Melatonin & Circadian Rhythm

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  • 0:05 Pineal Gland
  • 1:02 Melatonin
  • 2:01 Circadian Rhythm
  • 3:46 Seasonal Affective Disorder
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

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.

Pineal Gland

The pineal gland is the size of a pea and is located between the two hemispheres of the brain.
Pineal Gland

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.
Most Melatonin At Night

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.

Circadian Rhythm

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.
Light Shuts Off Melatonin

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.

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