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The epithalamus is another very important in the brain, located in the diencephalon, or forebrain. Within it are the habenular nuclei, pineal gland, and the stria medullaris.

These structures function as a part of the sympathetic nervous system and control the sleep-wake cycle (the Circadian rhythm), and, collectively, connect the limbic system to parts of the brain.

Not only does the epithalamus regular the Circadian rhythm, but it also plays a role in the regulation of emotions.

Habenular Nuclei

The habenular nuclei, which can also be referred to as the habenulae, form a part of the dorsal diencephalic conduction (DDC) pathway. The DDC is present in all vertebrates and works alongside two more fiber tracts, the stria medullaris (SM) and the fasciculus retroflexus (FR).

The SM sends neuronal transmissions to the habenulae from the forebrain, and the FR is primarily made of efferent nerves that send transmissions from the habenulae toward the midbrain and the hindbrain.

The habenular nuclei are situated in a pair next to the third ventricle (remember that the third ventricle is a part of the ventricular system that is responsible for creating cerebrospinal fluid and distributing it throughout the body).

Together, the habenular nuclei can also be referred to as the habenular complex and are divided into two main regions called the medial habenulae (MHb) and lateral habenulae (LHb).

The entire complex receives neural signals from the limbic system and basal ganglia. Based on these transmissions, the habenular nuclei send signals out to the specific targets in the midbrain (particularly the substantia nigra and tegmentum) to control the release of dopamine. Signals are also sent to the raphe nuclei, a collection of nuclei in the brainstem, to produce serotonin.

The control and flow of dopamine is a major component in the learning process, specifically for skills and concepts that can be enhanced by a reward system. When you are rewarded for learning a skill or concept, dopamine is produced in proportion to the level of satisfaction you may feel. For example, let’s say you are playing a video game:

When you are facing a horde of zombies and have to approach the situation stealthily (if you go in guns blazing your character will die), it may take a few tries to figure out the best strategy. With each try, the habenular nuclei are receiving sensory and cognitive inputs that influence them to produce dopamine.

As you progress in getting closer and closer to a safe zone, or even with the more zombies you shiv quietly, the dopamine levels increase. This enhances your cognitive ability to store that information for future use.

Dopamine is produced in amounts proportional to the expectation of the size and/or intensity of the reward. So, if there is an unexpected reward waiting for you in the safe zone, such as a new weapon, there will be even more dopamine produced, enabling you to recall the strategy you used to get around the horde. (However, this process does have a negative side.

When the production of dopamine is hyperactive this can result in a type of obsessive-compulsive behavior wherein you are too enthralled in reward-seeking behavior.)

On the other hand, when the reward is smaller than the expectation dopamine production is not just lesser, but actually inhibited. Interestingly enough, it has been found that the inhibition of dopamine production actually entails more activity from the habenular nuclei, suggesting that this structure is involved in passing on information about a lack of rewards.

It’s been found that the habenular nuclei are highly active in processing punishment and negative experiences, and has been found to play a role in disorders such as major depressive disorder.

Pineal Gland

Take a deep breath. In, out. And get ready to align your chakras. Why? Because the pineal gland was once known as the “third eye.” The philosopher Descartes called it the “principal seat of the soul and the place in which all our thoughts are formed.” Nobody agreed with him, but still.

Along with the habenular nuclei, there is still much to learn about the pineal gland. What is known currently, though, is that it plays a large role in the production and distribution of melatonin.

The pineal gland is located in the center of the brain, and shaped like a pine cone – where it gets its name (in Latin, pinea means “of the pine” or “covered in pines”). It is a type of endocrine gland that is known by many names apart from the third eye: the conarium, epiphysis cerebri, pineal organ, and pineal body.

The nerves that supply information to the pineal gland are highly sensitive to the hormone epinephrine – giving them their name, adrenergic nerves. It also functions in the reception of light (one of the reasons it was regarded as the third eye).

In addition to the reception and processing of light, the pineal gland has another function in common with the eye in that melatonin is also synthesized by the retina invertebrates. (The production of melatonin in the retina is influenced by the concentration of light in the environment and is even informed by receptors located in the skin and gastrointestinal tract.)

Melatonin produced by the pineal gland has been found to increase in concentration along with the setting of the sun and on into the darkness. These concentrations are also higher in the cerebrospinal fluid (CSF) that sits in the third and fourth ventricles of the brain, and in the blood as well.

It is thought that the melatonin present in the CSF may possibly have more direct and sustained effects on its target sites within the central nervous system.

For a moment, let’s return to the “third eye” concept for a second. To make the case for Descartes and try to get him a little redemption for his idea about the pineal gland being rejected by the world: The pineal gland is a part of a subsystem called the “photoendocrine system.”

What makes it apart of the photoendocrine system is the fact that noradrenergic nerves (sensitive to norepinephrine) terminate (end) in the pineal gland. These noradrenergic nerves work along with the retina and suprachiasmatic nucleus (in the hypothalamus) to gather information on light and inform the regulatory processes for the Circadian rhythm. (See! The pineal gland works along with the eyes – the windows to the soul. So… no, he was still wrong. Very wrong.)

Stria Medullaris

There isn’t much to say about the stria medullaris except that it is a bundle of fibers, or nerves, that are mostly efferent and directed toward the habenulae. This bundle is what forms the ridge on the medial (inner, toward the center of the body) surface of the thalamus.

The Circadian Rhythm

The Circadian rhythm is what you’ve probably heard referred to as your “biological clock.” Although, these two are not the same in that an organism’s biological clock is what creates a circadian rhythm.

Its effects are shown on the physical, mental, behavioral levels, in a cycle that spans over the entire 24-hour cycle, influenced strongly by light. What you probably didn’t know, though, is that it is also influenced by your genes!

Yep, that’s right – the regulation of your Circadian rhythm is influenced in part by your genetic history. Because it plays such a direct role in possible mental health disorders that either arise from or are exacerbated by sleep patterns, it makes sense that this cycle could be partially impacted by genetics.

The average person’s Circadian rhythm and the associated physiological occurrences play out similar to this:

  • 0600: Sharpest rise in blood pressure
  • 0700: Secretion of melatonin stops
  • 0800: High chance of a bowel movement
  • 0900: Highest level of secretion of testosterone
  • 1000: High alertness
  • 1400: Best coordination; Fastest reaction time
  • 1700: Greatest cardiovascular activity
  • 1800: Highest blood pressure; Highest body temperature
  • 2100: Secretion of melatonin begins
  • 2200: Suppression of bowel movements
  • 0200: Deepest sleep
  • 0400: Lowest body temperature

Scientists are seeing some dramatic changes in the way that the human Circadian rhythm is regulated, and that is largely due to the use of electronics, among other things. You know the feeling of lying awake in bed, waiting for yourself to fall asleep. So, to pass the time, you grab your phone and start scrolling through Instagram, only to realize that now you’re even more awake!

Well, this happens because of the light that is being projected from your phone, laptop, etc. The light is throwing off this natural rhythm that is outlined here and communicates to the pineal gland that it needs to get your body up and running. Many systems are then thrown off by this miscommunication, and not only is the timing of your sleeping schedule way off but so is your physiology as well.

Further, although you can adjust your sleeping schedule over time, being a “night owl” or a “morning person” isn’t entirely up to you. In fact, these behaviors are the result of your genetics as well, making them a kind of phenotype (expression of your genotype, or genes).

This is why it is best to choose work schedules and hobbies according to the time that works best for what you know to be true of your body (especially if it’s a choice between a daytime schedule or a graveyard shift) because there’s only so much you can do to fight your genetics.

The Circadian rhythm also affects your eating habits and digestion, and, apart from mental health conditions, has been found to play a role in many chronic health conditions like sleep disorders, obesity, and diabetes.

Now that you know exactly what regulates your Circadian rhythm, do yourself a favor and apply this knowledge to your life. Experiment with slight changes (within reason) in your sleeping patterns and note the physical and behavioral changes you might notice – what times of day, or which functions are affected the most? With this knowledge, you can help your “third eye” out by knowing exactly what affects the productivity and health-related aspects of your day.