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Along with playing an integral role in a healthy sleep-wake pattern, circadian rhythm has also recently been linked to health or poor physical and mental well-being.

Researchers are discovering and unraveling a unique relationship between the human circadian rhythm and two important hormones: cortisol and melatonin. Cortisol, also known as the stress hormone, helps us wake up and provides us with energy in the morning. Melatonin, on the other hand, brings our bodies down from the day and prepares us for sleep. These hormones work in tandem alongside our natural circadian rhythm.

Why is this so important? When any part of the relationship between these three factors becomes unbalanced, our bodies feel the pain of it. Imbalance can cause fatigue, anxiety and depression, and even unwanted weight gain.

So, let’s jump in and learn all about the circadian rhythm, melatonin, and cortisol!

Circadian Rhythm 101 + Terminology

Ever wonder why you get sleepy when the sun sets? How about why your body wakes you as the sun shines through your window in the morning? Have you ever contemplated why, in the dead of winter, all you want to do is curl up on the couch and take a nap?

Well, you can blame your circadian rhythm for all of it.

What is Circadian Rhythm?

Circadian rhythm refers to the physical, mental, and behavioral changes of the human body that respond primarily to the daily cycle of light and dark in our environment. Our bodies tend to get tired during dark hours due to a complicated and delicate relationship between our natural circadian rhythm and hormones.

Yet, your circadian rhythm plays many other roles as well. Along with influencing your sleep-wake cycles, the circadian rhythm is also responsible for “hormone release, eating habits and digestion, body temperature, and other important bodily functions.” When your circadian rhythm is disrupted, it can lead to “various chronic health conditions, such as sleep disorders, obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.”

Biological Clock v.s. Master Clock v.s. Circadian Rhythm

At this point, other similar terms may be popping up in your head, such as the biological clock or master clock. These are terms that our society is already familiar with and that researchers have spent many years unraveling. While all three of these biological functions work together to keep our bodies on a natural and healthy sleep-wake schedule, they are not the same thing.

A biological clock is an “organism’s innate timing device,” which is “composed of specific molecules (proteins) that interact in cells throughout the body.” Your biological clock is a tangible part of your human makeup — found in “nearly every tissue and organ” — while a circadian rhythm is a manifestation of your biological clock and the hormonal changes that are produced via environmental stimulants. Simply put, your “biological clock [produces] circadian rhythms and [regulates] their timing.”

With that said, both circadian rhythm and biological clocks exist in other living beings besides humans. While the genes related to biological clocks are found in “fruit flies, mice, fungi, and several other organisms that are responsible for making the clock’s components,” circadian rhythms have been identified in almost all living things “including animals, plants, and many tiny microbes.”

So, what is the master clock, and how does it fit into this delicate system?

Your master clock resides in the brain and is responsible for coordinating all the “biological clocks in a living thing, keeping the clocks in sync.” Just like the biological clock, the master clock is a tangible, identifiable part of the human body. Specifically, “in vertebrate animals, including humans, the master clock is a group of about 20,000 nerve cells (neurons) that form a structure called the suprachiasmatic nucleus, or SCN,” located in the hypothalamus, which “receives direct input from the eyes.”

So, in essence, the master clock coordinates the biological clock, the biological clock produces circadian rhythms, and the circadian rhythm controls our sleep-wake patterns. Yet, there’s another component to the whole system: hormones. Your circadian rhythm triggers two hormones that either wake you or make you sleepy. These are called melatonin and cortisol.

Melatonin v.s. Cortisol

In recent years, chronobiology, the study of circadian rhythm has led researchers to discover the essential connection between melatonin, cortisol, and our natural circadian rhythms. But first, we need to understand what melatonin and cortisol are. Here’s a quick introduction!

What is Melatonin?

Melatonin is a hormone that “plays a role in your natural sleep-wake cycle.” At night, your blood has the highest levels of melatonin, generally “at least 10-fold higher than daytime concentrations.” Melatonin is produced in the pineal gland, then “secreted into the blood stream and cerebrospinal fluid (the fluid around the brain & spinal cord) [where it] conveys signals to distant organs,” and is finally circulated “from the brain to all areas of the body.”

How does melatonin make you sleepy?

Melatonin-specific receptors — tissues expressing proteins — can “detect the peak in circulating melatonin at night and this signals to the body that it is night-time. While melatonin is regulated by your circadian rhythm, it also has “a season (or circannual) rhythm, with high levels in the autumn and winter, when nights are longer and lower levels in the spring and summer.”

What is Cortisol?

Cortisol is another important hormone in the human body. While you may have heard of this hormone referred to as the “stress hormone” — due to “its connection to the stress response” — cortisol has many other important functions.

Cortisol belongs to a group of hormones called steroid hormones, which are produced from the adrenal glands. Much like melatonin, cortisol is identified throughout the body by cortisol receptors and is “controlled by the hypothalamus, the pituitary gland, and the adrenal gland, a combination [of] glands often referred to as the HPA axis.”

Along with playing an important role in your circadian rhythm, cortisol can also help “control blood sugar levels, regulate metabolism, help reduce inflammation, and assist with memory formulation.” Plus, this mighty little hormone also has a “controlling effect on salt and water balance and helps control blood pressure.”

The Relationship Between Circadian Rhythm, Melatonin, and Cortisol

The relationship between your circadian rhythm and these two hormones — melatonin and cortisol — is a complicated one.

The sleep-wake cycle is determined by a few factors, but two of the most powerful are the circadian rhythm — also called process C — your homeostatic “sleep propensity” — also called process S. Your sleep propensity “determines the recent [accumulated] amount of sleep and wakefulness.”

Where do melatonin and cortisol come into play?

As mentioned above, melatonin and cortisol are regulated by the hypothalamus, specifically the suprachiasmatic nucleus (SCN). When it comes to your circadian rhythm (process C) and your sleep propensity (process S), your SCN is in the driver’s seat. In particular, the SCN promotes either suppression or synthesis and release of melatonin and cortisol, depending on your circadian rhythm.

Simply put, your glands produce melatonin and cortisol in response to environmental factors that are recognized by the brain. These hormones are then controlled by the suprachiasmatic nucleus in the hypothalamus — also called the “central circadian rhythm generator” — which makes you sleepy or wakeful and, in conjunction with your biological and master clocks, creates your circadian rhythm.

Habits to Harmonize Your Circadian Rhythm

While it may seem like you have no control over this delicate system between the environment, your brain, and your hormones, certain lifestyle habits can influence the efficacy of your circadian rhythm. As your sleep-wake cycle affects many parts of your health, implementing some of these habits can attribute to your overall quality of life.

Reduce Screen Time

You’ve most likely already heard about blue light, yet did you know that it negatively affects your entire sleep-wake cycle? Recently, research “has uncovered how light-sensitive cells in the eye can reset the internal clock when exposed to light. Due to artificial light from smartphones, tablets, laptops, and television, “our sleep-wake cycles are no longer tied to patterns of day and night.” This can upset our circadian rhythm, making it difficult to fall asleep and stay asleep. It’s recommended to avoid all blue light at least 30 minutes to an hour before bedtime.

Connect with Nature

Getting back to nature can get your circadian rhythm back to normal as well!

Recent studies highlighted that those with access to green space had higher activity levels and a higher quality of overall well-being, which had an indirect positive effect on the health of their sleep cycle. Plus, access to natural environments “could directly impact sleep by providing sufficient light and appropriate temperature for a healthy sleep-wake cycle.” On the other hand, it was found that “people with less access to natural amenities and green space were at a higher risk for feeling like they did not get enough sleep.”

Practice a Strict Sleep-Wake Schedule

While it may seem that you don’t have control over your circadian rhythm, enacting a strict sleep-wake schedule can help reset a badly tuned clock.

Per Dr. Stephen Amira, “Going to bed and waking up at the same time each day sets the body’s ‘internal clock’ to expect sleep at a certain time night after night. Try to stick as closely as possible to your routine on weekends to avoid a Monday morning sleep hangover. Waking up at the same time each day is the very best way to set your clock, and even if you did not sleep well the night before, the extra sleep drive will help you consolidate sleep the following night.”

How Plant-Based Eating Affects Circadian Rhythm and Hormonesquinoa breakfast bowl

Protein-Packed Breakfast Quinoa Bowl/One Green Planet

Hormones are naturally finicky little chemical messengers that coordinate, control, and influence almost all systems in the human body. When off-balance, they can disrupt these systems in negative ways, and there’s a laundry list of reasons, including high-stress levels, poor gut health, exercise habits, and lack of sleep, to name just a few! This is true for all hormones, including the essential circadian rhythm influencing melatonin and cortisol.

So, how does diet play a role in balancing these essential hormones? It’s all connected. Especially when it comes to your endocrine system, when one hormone is off-balance, this can cause a cascade effect on the rest of your hormones.

The foods that you consume determine how efficiently your body works, including the smooth running of your endocrine system. Specifically, the “energy and nutrients you obtain from your diet are the raw materials your body needs to produce hormones and properly fuel your body… If your diet doesn’t supply enough energy or ‘materials’ to make all the hormones you need, it’ll prioritize production of stress hormones first because they’re essential for survival.”

The key is to integrate a nutrient-dense, low-processed, organic diet that is naturally filled with a spectrum of vitamins, minerals, and macronutrients.

Luckily, plant-based eating fits all of these categories!

Along with getting the appropriate protein and carbs, for optimal hormone health you’ll want to focus on probiotic-rich foods — Raw Fermented Coconut Yogurt or Pickled Green Tomatoes — prebiotic-rich foods — Banana Nut Whole Grain Scones or Chicory Polenta with Caramelized Chicory — fiber-rich food — Lentil Stew or Protein-Packed Breakfast Quinoa Bowl —and healthy fats — including olive oil, coconut oil, nuts, and seeds.

With that said, before changing up your diet, make sure to speak with your doctor.

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