Circadian Rhythms

Written by main_admin on August 24, 2020


Many life forms follow certain patterns of behavior that are linked with the day and night cycles of the planet. Virtually all animals have some sort of a sleeping routine. There’s also evidence for the presence of internal clocks that regulate the body’s biological processes based on the daytime. However, these biological rhythms appear to be present even in the absence of external cues, such as light and temperature.

In 1729, a French scientist Jean-Jacques d’Ortous de Mairan saw how the 24-hour patterns in the movement of the leaves of Mimosa pudica remained constant even when the plant was kept in continuous darkness…

These endogenous cycles were first independently discovered in fruit flies in 1935 by two German zoologists, Hans Kalmus and Erwin Bünning.

The term “circadian” was derived from circa (about) and dies (day); it may serve to imply that certain physiologic periods are close to 24 hours, if not exactly that length. “Circadian” might be applied to all “24-hour” rhythms, whether or not their periods, individually or on the average, are different from 24 hours, longer or shorter, by a few minutes or hours.

In 1977, the International Committee on Nomenclature of the International Society for Chronobiology adopted the official definition for circadian rhythms, which goes like this:

Circadian: relating to biologic variations or rhythms with a frequency of 1 cycle in 24 ± 4-h; circa (about, approximately) and dies (day or 24 h).

In 2017, three scientists Jeffrey C. Hall, Michael Rosbash, and Michael W. Young were awarded the Nobel Prize in Physiology or Medicine for their discoveries of molecular mechanisms controlling the circadian rhythm.

During their research, the men identified a gene in fruit flies that controls their circadian rhythms. They named this gene period, which encodes a protein called PER. PER accumulates during the night and degrades during the day, thus it oscillates over a 24-hour cycle, in synchrony with the circadian rhythm.

Circadian rhythms enable living organisms to prepare for and adapt to environmental changes that happen on a regular basis.

For instance, the coming winter, night-time, seasonality of certain food sources, and fluctuations in climate. By now we know that almost every organ and individual cell has its own biological clock that’s regulated by the master clock in the brain.

In regards to health and nutrition, circadian rhythms play an immense role. They’re going to dictate what kind of physiological processes to commence and which hormonal patterns to follow. That’s why it’s a vital component to the regulation of autophagy, growth hormone, fat loss, insulin sensitivity, and much more.


Our bodies operate on a circadian rhythm, which is defined by the National Institutes of Health (NIH) as “the natural cycle of physical, mental, and behavior changes that the body goes through in a 24-hour cycle.” The circadian rhythm impacts much of our body’s functions, from hunger to sleep patterns to hormonal fluctuations.

Think of the circadian rhythm as a pacemaker for your body. The brain’s master circadian clock (which controls your body’s circadian rhythm) is made up of tens of thousands of cells located in the hypothalamus. As our eyes perceive light, this master clock sends hormones—mainly cortisol, to wake you up, or melatonin, to make you sleepy—to every cell of your body to keep everything in sync.

This complex timekeeper is controlled by an area of the brain that responds to light, which is why humans are most alert while the sun is shining and are ready to sleep when it’s dark outside.

It is not at all natural circadian rhythm for humans to be awake after midnight and sleep until noon. Those things are the result of living in a modern world with different types of circadian disruptors and lifestyle factors. Shift work, playing video games until the morning, and partying under a fluorescent light is unnatural and one of the worst things for your health.

If you follow your body’s natural cues regarding when to go to sleep and wake up, your circadian rhythm should stay balanced, but a change in your schedule (like if you stay up late pulling long hours at work one day or sleep in one Saturday), can disrupt your body clock. Follow these three tips to keep your circadian rhythm functioning as it should.

1. Stick to a Consistent Sleep Schedule. A regular bedtime is one part of the equation, but waking up at the same time daily will also help keep your circadian rhythm in check. It may be tempting to grab some extra shut-eye on weekends, but doing so can throw off your body clock during the week.

2. Go for an A.M. Walk. In the morning, exposure to the sun (or indoor light), won’t just give you an energy boost—it can also reset your circadian rhythm. A quick outdoor stroll in the morning will give you enough sun exposure to signal to your brain that it’s time to start the day. No time to walk? Simply raise the blinds and find a way to expose yourself to sunlight.

3. Limit Evening Tech. Bright blue lights emitted by screens in the evening hours can throw off your body clock by confusing your brain into thinking it’s still daytime. Artificial blue light (the type that laptops, tablets, and cell phones emit) is the worst culprit, so try to power-down tech devices at least two to three hours before bed. If you must work during night hours on your laptop, consider downloading an app called “f.lux” that warms up your computer display at night, to match your indoor lighting.


There are 3 main signaling factors that affect the circadian rhythms – light, movement, and food. Most of the circadian signaling is transmitted through your eyes. When light enters the retinas and gets transmitted into the brain it stimulates the suprachiasmatic nucleus (SCN). The SCN is the master circadian clock in your body that regulates all the other biological rhythms and clocks. There are many different types of clocks and it’s thought that most organs like the liver, heart, and pancreas actually have their own circadian clock. That’s why all these different factors like sunlight, physical exercise, and eating affect the entire circadian rhythm of your body.

Light is made of many electromagnetic particles or photons that travel through space in a wavelength form. They emit energy and are represented by different colors. Sunlight’s wavelength is called the solar spectrum and it contains ultraviolet, visible, and infrared wavelengths.

The human eye can only detect visible light which is seen as either violet, indigo, blue, green, yellow, orange, or red light. Blue light is part of the natural environment and can be seen almost everywhere. The reason why the sky is blue is actually that the blue light coming from the Sun collides with the air molecules and makes the blue light scattered everywhere.

Blue light exposure to the eyes plays a very important role in regulating your circadian rhythms and day and night cycles. It has antibacterial properties, boosts wakefulness, increases alertness, and can adjust the circadian clock. Too much blue light at the wrong time can damage your mitochondria, promote insulin resistance, cause insomnia, depression, and increase inflammation.

Blue light has a short wavelength (380-500 nM), which makes it produce higher amounts of energy. Naturally, you wouldn’t get exposed to much blue light aside from the early to afternoon parts of the day. However, ever since the invention of the light bulb, our environment has many additional sources of blue light. Because of technology and new gadgets, we’re getting exposed to more blue light for longer periods of time which can offset the circadian rhythm and cause damage to our health.

Blue light exposure at night and circadian mismatches are linked to many types of cancer, diabetes, obesity, heart disease, and Alzheimer’s. You wouldn’t think that it has such a huge role but after you learn about how light affects your body’s biological processes you’ll realize how serious this actually is.

Melatonin is the sleep hormone and a powerful antioxidant that helps to conduct many repair processes in the brain and body. If you inhibit melatonin secretion because of blue light at night, then you’re going to lower growth hormone, which makes it more difficult for you to burn fat and build muscle, and you’ll also prevent the brain from clearing out the toxins that get accumulated there during the day.

Protect Your Circadian Rhythms

However, not all blue light is bad. The timing of when you get exposed to it matters a whole lot more, which is why you’d have to entrain yourself to follow a proper circadian rhythm.

– Expose yourself to the natural sunlight first thing in the morning. This will synchronize your biological clock to the surrounding environments and maintains consistency.

– If you live in an area where you don’t get much sunlight or if it’s cloudy, then use blue light-emitting devices such as the Human Charger or face lamps for 10-15 minutes.

– On days with clear sunlight try to spend more time outside by going for a long walk. This will raise vitamin D levels and charges up the mitochondria as well. The majority of circadian signaling happens through the eyes so try to expose them to the daylight. Don’t wear sunglasses or hats that cover your vision either because you’ll miss out on the blue light.

– When indoors wear long-sleeve clothes to protect your skin from too much blue light exposure. A lot of the circadian signaling also happens through the skin, which is why you don’t want to sit under fluorescent lights before bed.

– Wear blue-blocking glasses in the evening.

– Sleep in pitch-black darkness with blackout blinds and a sleeping mask that covers your eyes.

– Make sure there are no hidden sources of blue or green light in your house like the alarm clock, night lamps, red dots on the TV screen, smoke detector lights, etc.


The concept is part of a larger movement in the health and wellness communities towards “circadian” health, as coined by the Global Wellness Institute. “While intermittent fasting is all the rage, people don’t realize that this is also usually a circadian-based solution,” reads the institute’s 2020 trends report. “It’s natural for diurnal animals such as humans to eat during daylight when we evolved to digest food.”

So, should we all start thinking of intermittent fasting as circadian rhythm fasting? Here’s what you should know, straight from two experts who study circadian rhythms.

The most recent study also involved mice. While the animals adhered to 24-hour periods of fasting, the scientists measured various physiological functions.

They saw that while fasting, mice used less oxygen and energy. However, as soon as the mice ate, these gene-driven physiological changes were reversed. This mirrors what researchers have previously seen in humans.

Lead study author Prof. Paolo Sassone-Corsi explains what the researchers found, saying, “We discovered [that] fasting influences the circadian clock and fasting-driven cellular responses, which together work to achieve fasting-specific temporal gene regulation.”

They also note that it influenced different tissue types to different degrees. As Prof. Sassone-Corsi says, “Skeletal muscle, for example, appears to be twice as responsive to fasting as the liver.”

It seems logical that intermittent fasting may be able to support your circadian rhythm. However, it’s important to remember that a lot of the research around intermittent fasting and circadian rhythms is still in the early phases. Many of the studies are either small and short-term (meaning that they looked at just a few subjects for a short period of time) or they’re on mice— which can be a good foundation for future knowledge but doesn’t necessarily mean that the conclusions are true in humans, too.

Dr. Lockley says that your body starts to produce melatonin at night a few hours before bed to help you feel sleepy—and eating during this time disrupts melatonin production, which then disrupts your sleep.

“Nobody knows how many hours [of fasting] is healthy,” says Dr. Lockley. Which is why it may be more beneficial to focus less on how many hours you go without eating, and more on timing your meals to your circadian clock. “It’s not about restricting. It’s about getting back to a more natural cycle where we don’t eat at night,” he says.


Another critical component of circadian rhythms is food intake. It turns out that the timing of when you eat is even more important to your health than what and how much you eat.

Time-Restricted Feeding (TEF) emerged as a concept within the context of circadian rhythms. The master clock is very much connected to nutrient-sensing pathways that detect the presence of calories and food

In most animals, feeding is confined to a certain time period, which leaves a short period of fasting that coincides with sleep. Unfortunately, modern life not only disrupts our circadian rhythms with light but also with food. An average person in the Western world tends to spend most of the day in a fed state, leaving no time for the body to heal itself. Some people can even eat right before going to bed, sleep for about 7-8 hours, and start eating immediately after waking up. This prevents them from ever entering into fasted conditions that are so vital for longevity.

Dr. Satchin Panda, who is a professor at the Salk Institute and an expert in circadian rhythms, recommends people to eat their food within a minimum of 8-10 hours. During fasting, the gut and immune system have then enough time to repair themselves and conduct other autophagic processes. Dr. Panda’s research has found that the average person eats over a 15-hour period, starting with a drop of milk in their morning coffee and ending with a late-night snack of some nuts or chips.

In 2015, a study tested how eating an entire day’s caloric intake within 10-11 hours affect overweight individuals. Their eating window ended around 8 PM. They lost about 4% body weight in 16 weeks and retained it for up to a year.

This was accompanied by a spontaneous 20% reduction in calories just because of skipping out on random snacks or alcohol late at night. The participants also reported improved sleep and higher alertness during the daytime.

Time-restricted feeding has also been shown to prevent metabolic disorders in mice who are fed a high-fat diet without reducing calories. The mice who were fed their food within 8 hours didn’t get obese or develop the disease compared to those who ate the same amount of calories with no time restrictions


There’s another important signal that keeps our circadian clocks in sync: the timing of meals. A very small 2017 study had 10 men get used to eating early meals for five days, then switched them to eating later in the day for six days. They found that the late meals delayed the functioning of PER2, a gene that’s that helps regulate the circadian clock.

Dr. O’Neill oversaw a 2019 study in animals that could give some insight as to why meal timing impacts the circadian rhythm and therefore can be important for health. Essentially, his research found that insulin (the hormone that helps regulate blood sugar) played a role in resetting the circadian clocks in mice; when insulin was mistimed, it disrupted the circadian rhythms of the mice.

Mice are of course very different from humans. But in theory, this is how the relationship between food and circadian rhythms could play out in people, says Dr. O’Neill.

When you eat breakfast, you’re breaking an overnight fast with a meal that stimulates your pancreas to pump out more insulin. This hormone tells your cells to store glucose—which controls blood sugar levels and provides your cells with energy—as well as to make more of the PERIOD protein that enables all of your individual cells to “keep time” according to the circadian rhythm and stay in sync with each other. This is supposed to happen every day to ensure your cells are all working on the same schedule. “[Glucose] does the business end of the timing mechanism,” says Dr. O’Neill.

Meanwhile, mistiming (as in, eating at the wrong time of day) appears to be harmful to our health. “When we eat at night, our bodies can’t cope as well,” Dr. Lockley says.

Research has found that shift workers, who are awake when it’s dark out and eat at unusual times of the day, are more susceptible to health problems including obesity and cardiovascular disease. “We think it’s because cortisol and insulin signals get disrupted in relation to each other,” says Dr. O’Neill. When you eat a large meal very late at night, your body is being told to produce a lot of insulin during a time of day during which it’s used to resting, says Dr. Lockley. When you’re resting, your body doesn’t need to use glucose for energy (your body prefers to burn stored fat at night, Dr. O’Neill says) so you end up with a surplus of glucose hanging out in your bloodstream. This can impact the quality of your sleep, your mood and energy levels, and your eating habits.

One, when you fast overnight your body switches from burning glucose for energy to burning stored fat. Additionally, “your body seems to anticipate that you’ll have nutrients to store during the day and liberate at night,” says Dr. O’Neill. The result? When we’re active during the day but eat at night, our bodies metabolize food less efficiently, so we’re less likely to switch into fat-burning metabolism.

This is where intermittent fasting comes in—specifically 16:8, an iteration of IF where one eats during an eight-hour window every day and then fasts for 16 hours overnight. Although Dr. O’Neill hasn’t yet studied the effects of intermittent fasting itself on health, he says his 2019 findings support the benefits of this particular form of intermittent fasting. Essentially, eating during these specific windows of the day is in line with your own circadian rhythms—which, in theory, can make for healthier blood sugar levels, better weight management, improved sleep, and more.

As a functional medicine doctor who is known for preaching a food-as-medicine message, Mark Hyman, MD, is used to patients asking him all sorts of IF questions—or coming to him complaining that it hasn’t worked for them. Dr. Hyman says the new information makes sense and even says IF is linked to a myriad of health benefits. “Intermittent fasting, or eating your meals within an eight-hour window can be effective, especially in terms of changing unhealthy eating behaviors,” he says.

But he also says he’s seen people go about IF the wrong way, too. “I think people get caught up in the terminology of it, not knowing what intermittent fasting really is,” Dr. Hyman says. “If you think about it, having your last meal by 9 p.m. and not eating again until 10 a.m. is a version of IF—that’s why the morning meal is called ‘breakfast’; you’re breaking a fast. But if you fast from 9 p.m. to 10 a.m. and then essentially eat all day, snacking between meals, well, you’re not going to see the benefits.”

In other words, fasting for a certain period of time doesn’t mean mindfulness shouldn’t come into play outside your fasting window; what you eat matters all the time. Intermittent fasting may help cut down on mindless midday or late-night snacking—part of that behavior change Dr. Hyman points out that can lead to real benefits—but it’s still important to fill up on healthy, whole foods whenever you do sit down to mealtime.

It’s just another piece of evidence showing that consistently eating healthy, reasonably-portioned food is what really works— whether you’re intermittent fasting or not.

Here are answers to some of the most common questions about intermittent fasting:

1. Can I drink liquids during the fast?

Yes. Water, coffee, tea, and other non-caloric beverages are fine. Do not add sugar to your coffee. Small amounts of milk or cream may be okay, but to get the most out of fasting we recommend avoiding it.

2. Isn’t It unhealthy to skip breakfast?

No. The problem is that most stereotypical breakfast skippers have unhealthy lifestyles. If you make sure to eat healthy food for the rest of the day then the practice is perfectly healthy.

3. Can I take supplements while fasting?

Yes. However, keep in mind that some supplements like fat-soluble vitamins may work better when taken with meals.

4. Can I work out while fasting?

Yes, fasted workouts are fine. We recommend taking branched-chain amino acids (BCAAs) before a fasted workout.

5. Will Fasting Cause Muscle Loss?

All weight loss methods can cause muscle loss, which is why it’s important to lift weights and keep your protein intake high. One study showed that intermittent fasting causes less muscle loss than regular calorie restriction.

6. Will Fasting Slow Down My Metabolism?

No. Studies show that short-term fasts actually boost metabolism. However, longer fasts of 3 or more days can suppress metabolism.

7. Should Kids Fast?

Allowing your child to fast is probably a bad idea.

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