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Article authored by Readout Health with editorial oversight from Chief Medical Officer, Naomi Parrella, M.D.

Ever wonder why when the sun sets and darkness falls, you suddenly can’t help but sneak a yawn? Or why when the morning sunlight hits your face, you start to perk up? This is no coincidence. If it seems like these things happen like clockwork, that’s because it does. Your body is actually hardwired to follow a daily circadian rhythm, and it’s this rhythm that influences how alert you are, how hungry you are, and how energetic you feel.

Circadian rhythms and the body’s master clock

Circadian rhythms are the physical, emotional, and behavioral changes that occur in the body over a 24-hour cycle. It affects not only humans, but also most living things including animals and plants. It impacts practically every physiological process in the body and brain, influencing how every cell, organ, and tissue works. 

The sleep-wake cycle and the body-temperature cycle are two great examples of processes that rely on the body’s circadian rhythm. Darkness cues your brain that it’s time for sleep and to make more melatonin so that you feel drowsy, while morning sunlight notifies your brain that it is daytime, rousing you awake and keeping you alert. Likewise, body temperature follows a strict circadian rhythm, staying within a tight range throughout the day. During the daytime, body temperature increases, priming your immune system to fight off harmful invaders, and falls back down at night while you’re at rest. 

What exactly coordinates this “master clock”? The answer lies within the organ that plays a role in just about every major bodily process: the brain. Within the brain, 20,000 nerve cells form a structure called the suprachiasmatic nucleus, or SCN. Known as the body’s “pacemaker,” the SCN is located in the brain’s hypothalamus and coordinates most of the circadian rhythms, or biological clocks, around the body. The brain serves as the master clock, but virtually each organ and tissue contains its own biological clock, as well. These clocks that exist outside the brain are called “peripheral clocks.”

Both internal and environmental factors play important roles in influencing the body’s circadian rhythms. Genes contain the information needed to make proteins that activate feelings of sleepiness, alertness, and wakefulness at different times during the day. The SCN also receives direct input from the eyes. That is, incoming light that you see stimulates the eye’s optic nerves, directing the body’s central circadian rhythm. So, environmental exposure to light at different times of day can impact when the body turns on these genes.

What can dysregulate the body’s circadian rhythm? 

Modern day society doesn’t always make it easy to maintain a regulated circadian rhythm. A string of late nights working or a bout of jet lag is often enough to throw a circadian rhythm off. Other external factors like exposure to artificial light at night from electronic devices, alcohol consumption, or out of sync eating times can also disrupt the body’s circadian rhythm. And sometimes, certain genes can mutate or change in a way that impacts rhythm as well.  

Circadian dysregulation’s effects on health 

It’s easy to get caught up in the whirlwind of life – forgoing regular sleep, skipping meals, and spending a bit too much time scrolling your feed right before bed. But it’s worth paying attention to. Disruption of the body’s circadian rhythm can potentially impact health, resulting in metabolic syndrome, type 2 diabetes, obesity, impaired glucose tolerance, depression, cancer, intestinal disorders, and sleep disorders. In fact, research suggests that night shift workers, who are exposed to more artificial light and irregular light-dark patterns during the night, are at an elevated risk for disease. 

Night shift workers are also at higher risk for increased blood glucose levels. In one study, healthy subjects were randomly assigned to either a regular sleep schedule, where they slept from 11 p.m. to 7 a.m. from day 1 to 7, or a misaligned sleep schedule, where they slept from 11 p.m. to 7 a.m. from day 1-3, but were then shifted 12 hours to sleep from 11 a.m. to 7 p.m. for days 4 to 8. For the group that was on the regular sleep schedule, glucose tolerance lowered as expected over the course of the day from breakfast to dinner. But for the group who experienced a shift in sleep time, glucose tolerance was lower all around, with a 6% increase in post-meal blood sugar. And what about those who aren’t night shift workers? Research shows that exposure to dim light even while asleep is shown to negatively impact sleep duration and quality, hamper melatonin production, and dampen alertness and cognition. 

A disruption to circadian rhythm and weight gain

Changes to glucose tolerance isn’t the only thing to watch out for when it comes to a circadian rhythm that’s out of whack. Along with that can come weight gain, as several recent studies have found. 

One study in particular looked at how the timing of meals might affect weight when rhythm is shifted. The researchers explain that aside from light exposure, feeding time plays a critical role in regulating the body’s circadian rhythm, as humans are naturally adapted to ingest food – and elicit the release of insulin – in the daytime. The master clock is involved in a variety of metabolic rhythms, dictating insulin sensitivity and how much energy the body uses up during digestion. The peripheral clocks work during digestion, as well. The clock in the gut regulates the absorption of glucose, the clocks in the liver and fat tissue regulate blood sugar levels while fasting and the metabolism of lipids, and the clock in the pancreas regulates the secretion of insulin in relation to meals. The researchers found that both insulin and Insulin Like Growth Factor -1 (IGF-1) are synced with the body’s circadian rhythms and can act as a timing signal to cells throughout the body. These clocks all work together, and their ability to do so is dependent on the availability of food. Eating late at night throws off the body’s digestive clock rhythm and the release of insulin, potentially resulting in negative metabolic effects like weight gain. 

The idea that nighttime eating can dysregulate circadian rhythm and lead to metabolic conditions was confirmed in a 2018 study, where researchers studied the association of late night meals and snacking with metabolic syndrome among 8153 subjects. They found that snacking at night after dinner was linked with metabolic syndrome, including weight gain, type 2 diabetes, and high blood pressure in women, whereas both men and women saw an elevation in cholesterol as a result. Another study analyzed the possible effects that food timing can have on circadian rhythm. The eating patterns and weight of 1961 subjects were followed for three years. Results found that those who consumed most of their food later in the day, at lunch and dinner, saw the most weight gain when compared to those who ingested the most food earlier.

Aside from when a meal or snack is eaten, it also appears that the timing between meals can play a role in causing rhythm-dysregulated health conditions. Research from 2019 analyzed the effects of fasting and meal timing on health, comparing the outcomes of a high meal frequency – six or more meals per day – to low meal frequency, where only one or two meals were eaten. The researchers suggested that not only was fewer meals a day associated with lower BMI, but a longer overnight fast resulted in a lower BMI, as well. They attributed these effects to, among other things, an improvement of the circadian clock. 

Light and the timing of meals are both known contributors to the regulation of circadian rhythms, but nutrients and diets appear to affect biological rhythms as well. For example, low carb diets and their influence on rhythms and metabolism have been studied.  Restricting carbs in the diet or following a prolonged fast results in an insufficient amount of glucose in the body. This elicits the liver to produce ketone bodies to serve as an important primary fuel source, especially for the brain. Ketone bodies are tightly regulated by daily circadian rhythms.  Research suggests that ketone bodies can actually promote weight management by increasing clock gene expression and suppressing appetite.

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