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A woman in bed checking her alarm clock.

How in tune are you with the patterns of your day-to-day schedule? It turns out, your eating, exercise, and lifestyle efforts are not the only determinants of health: Your circadian rhythm is also key.

Circadian rhythm refers to the roughly 24-hour clock that governs much of your biology, including your sleep, behavior, body temperature, and hormone levels. This occurs through oscillating patterns that release certain proteins that influence how the body functions.

While most people are familiar with how circadian rhythm helps you feel awake during the day and sleep soundly at night, its effects extend well beyond that.

This “central clock” is governed by the body’s suprachiasmatic nucleus, or SCN. The SCN is a group of approximately 20,000 nerve cells found in the hypothalamus at the base of your brain. Additional mini clocks are found in peripheral tissues, such as your retina, intestines, liver, and muscle, too.

While genetics play a role in setting your “clock,” your circadian rhythm also requires external cues to help reset it daily.

The Health Effects of Circadian Rhythm

Your circadian rhythm is intimately tied to many key body systems, and several innate aspects of your health influence your ability to maintain a steady cycle.

Meanwhile, external factors — such as light exposure, exercise, and meal timing — affect your rhythm as well. These are referred to as “zeitgebers.” Travel, shift work, aging, and irregular schedules can also mess with your rhythm.

How rhythmic or not your circadian cycle is can have wide-ranging effects on your health, including the following:

Heart Health

Key functions of cardiovascular health work in a rhythmic pattern — and disruption to that rhythm can have adverse consequences.

Even short-term shifts can affect your blood pressure and your body’s ability to pump the breaks on your nervous system (or, activate a “rest and digest” return to homeostasis after a stress response). One prominent example is the increased risk of heart attacks that occurs immediately after Daylight Savings Time.

In the morning, circadian rhythm variation causes changes to hormones (such as cortisol and epinephrine) and body processes (such as heart rate and clotting factors). These variations are hypothesized to be the reason that heart attacks are more likely to occur between 7 a.m. and 1 p.m. than other times of the day.

Disruptions in sleep caused by a wonky circadian rhythm can also contribute to hardened blood vessels.

Asthma

Asthma is the result of inflammation in the airways (bronchial tubes), which causes difficulty breathing.

When it comes to circadian rhythm, several hormones and body processes follow what’s called a “diurnal” pattern, meaning that levels peak and valley at certain times during the 24-hour cycle. In asthma, inflammation follows a diurnal pattern and can peak in the early morning hours, making the risk of asthma attacks at that time of day greater.

Studies have concluded that the disease path of asthma is closely linked to the circadian rhythm of certain inflammatory pathways. Gaining a better understanding of this pattern can help with more effective disease management by timing medications — this is called chronotherapeutics. 

It’s been found that specific timing of certain asthma medications can result in better efficacy of those medications, even when the dose is the same. The dosage timing in these cases aligns with the diurnal pattern, which can help support a healthier circadian rhythm and inflammatory pathways, and in turn, may help minimize the occurrence of asthma attacks.

Blood-Sugar Control

When our central clock, or SCN, malfunctions, our bodies don’t regulate calories appropriately, and poor blood-sugar control can result.

Insulin resistance is a condition that typically precedes the development of type 2 diabetes. It occurs when cells don’t have an appropriate response to insulin, which works to lower blood sugar. Because blood-sugar levels are disrupted when our circadian rhythms are, disturbances to your central clock may directly lead to insulin resistance, and the subsequent development of type 2 diabetes.

Sleep Disorders

An out-of-sync circadian rhythm is a common cause of disrupted sleep, and there are several specific disorders with known links to the central clock. These are known as circadian rhythm sleep disorders (CRSDs), which can develop when the timing of your central (biological) clock is mismatched with external cues, or when there are changes in the activity of your SCN itself.

Common CRSDs include the following:

  • Delayed sleep phase disorder (DSPD): Sufferers have difficulty with both falling asleep and waking up. It’s estimated that up to 10 percent of all insomnia cases are from DSPD.
  • Advanced sleep phase disorder (ASPD): This is characterized by challenges staying awake the latter half of the day and maintaining sleep into the morning hours.
  • Irregular sleep-wake rhythm disorder (ISWRD): In this disorder, which is marked by daytime sleepiness and nighttime insomnia, the SCN is less active. Risk factors include traumatic brain injury, Alzheimer’s disease, and mental disabilities.
  • Non-24 hour sleep-wake phase disorder (N24HSWD): Sleep is typically delayed 1 to 2 hours, which creates difficulty sleeping and drowsiness during the day. In those struggling with this disorder, which is mostly experienced in people with blindness, the central clock has its own timing altogether; the process is referred to as a “free runner.”

With all of these conditions, there’s a heightened risk for abnormal eating due to an exaggerated mental reward system that occurs in response to food.

Psychological Health and Behavior

Mood disorders are increasingly common, and are often marked by a reduced size of the hypothalamus, the part of the brain that houses your central clock, or SCN.

Studies in mice show that disruptions in this master clock can lead to helplessness, despair, and anxiety-like behavior. Negative implications on conditions such as bipolar disorder, ADHD, and depression are all associated with an imbalanced 24-hour clock as well.

In those with bipolar disorder, sleep disturbances as a result of circadian rhythm disruption often increase just before a relapse or during an episode. Those with bipolar disorder have also been shown to have an exaggerated loss of melatonin — the hormone that promotes sleep — in response to light exposure.

Cortisol, the body’s stress hormone that follows a diurnal rhythm, has distinct shifts between manic and depressive states as well.

Those who struggle with depression often experience a higher-than-normal body temperature at night, which is typically regulated by the SCN. They also typically have decreased melatonin, which can affect their ability to get to sleep.

In addition, there are several genes involved in the pathology of major depressive disorder, and these genes have been found to change expression with alternations in the circadian rhythm.

Cancer

More than a decade ago, the World Health Organization classified shift work as a carcinogen. The disruption to our health and metabolism that occurs from working at night and sleeping during the day are largely due to alterations in circadian rhythm and sleep/wake patterns.

Since the body’s master clock can turn some metabolic controls on and off, imbalances in this rhythm have been tied to accelerating the growth of cancerous tumors.

For instance, some studies have shown that chronic exposure to light at the wrong times (an external cue that can disrupt your circadian rhythm) can even play a role in the development of breast cancer.

On the other hand, if functioning optimally, the SCN can play a role in addressing DNA damage and preventing the growth of abnormal cells. There are proteins, called REV-ERB proteins, that are part of the entire circadian system that can even potentially help prevent cancer cells from growing out of control.

Body Fat and Obesity

Excess fat storage and obesity are both incredibly complex. However, the metabolic process of these conditions have been linked to several hormones that are released by fat tissue and affected by circadian rhythm. One of these key players is leptin, the hormone that tells you that you feel full.

In healthy individuals, leptin levels have a set pattern. But it’s been found that in those with obesity, there’s less of a variation in leptin pattern, which dysregulates appetite and the ability to feel satisfied with food.

Another notable hormone related to fat storage is adiponectin, which is currently being explored as part of a potential treatment plan for obesity. Adiponectin, like other hormones with circadian rhythm, has a diurnal pattern. Disruptions in adiponectin cycling could also contribute to trouble with excess body fat.

Pain

Those who struggle with chronic pain often report that it has a cyclical nature, tending to worsen at certain times of the day. While it varies from person to person and by condition, studies have shown that the severity of pain can in fact be influenced by circadian rhythm changes.

Associations have also been made between circadian rhythm and migraines and cluster headaches. One study in Italy demonstrated that terminally ill patients who were on morphine had peaks in experiencing breakthroughs in pain concentrated during the midday, around 12:30 p.m. This indicated that there was a set pattern that could be linked to a 24-hour cycle and circadian rhythm.

Fertility/Labor

Since circadian rhythm affects hormone regulation, it’s not surprising that it can also effect fertility and labor.

Ovulation and egg release, sperm production, and even pregnancy implantation and maintenance are all interrelated with circadian rhythm. In those undergoing fertility treatments, it’s been shown that positive clinical pregnancy outcomes can be supported when sleep and 24-hour schedules are optimized.

Labor and delivery are never predictable, but observational analysis does demonstrate that the most common time for delivery is between 2 a.m. and 5 a.m. While there are several theories around why this timing is more common, despite disrupting the usual sleep-wake cycle, the trend and loose pattern of predictability indicates that the process is likely to have governance from the central clock. The practice contractions that often occur during the third trimester to help the uterus prepare for labor tend to follow a nighttime pattern, too.

Are you more of a morning or night person?

Historically, chronotypes included two designations: You were either a morning person or a night owl.

Recently, however, additional categories have been identified. Now, we also recognize those who are most alert in the afternoon, as well as those who are most alert both first thing in the morning and the evening, with a lull around midday — the latter folks are characterized as nappers.

Regardless of individual chronotype, we can all follow much of the same advice for setting and resetting our circadian rhythms for optimal health.

How to Optimize Your Circadian Rhythm

While many of us struggle with an off-kilter circadian rhythm, we can use certain external influences — or zeitgebers — to optimize our 24-hour schedule.

Be mindful about your exposure to light.

Exposure to bright light during the day — and avoidance of light at night — is arguably the most effective zeitberger to support a healthy circadian rhythm.

Aim for sunlight exposure first thing in the morning. When that’s not possible, the right type of light therapy might be a secondary option. Studies have shown that exposure to 2,000 to 2,500 lux of light therapy can help with certain sleep disorders and alleviate symptoms of major depressive disorders.

Conversely, blocking light at night is a nonnegotiable. This can be challenging in today’s world, as we’re often exposed to artificial light and screens late into the evening.

One small study on young men showed that LED light (common in computer screens) in the evening decreased melatonin (the hormone that regulates our sleep-wake cycle) and reduced sleepiness as compared to non-LED light. It’s worth looking into using incandescent or halogen bulbs in the evening (instead of CFLs and LEDs) if reducing all artificial light exposure is not doable.

As much as you can, do your best to avoid screens at least an hour prior to bedtime. If that’s not realistic, there are blue-light blockers and software that change the type of light emitted.

To support a healthy, pitch-black sleep environment, consider using blackout curtains or wearing an eye mask at night.

Prioritize your sleep.

Optimizing your sleep schedule to the best of your ability is a no-brainer when it comes to nearly all aspects of your health, including supporting your circadian rhythm.

Consider setting both a regular bedtime and wake time so you can maintain as normal and consistent of a sleep-wake pattern as possible. (Ideally, design this so you get at least 7 to 8 hours of sleep each night.) This means going to bed and getting up at the same time on weekdays and weekends.

This is critical in all stages of life: It’s been shown that parents who have created a set bedtime can improve health outcomes in their children.

If you tend to rely on naps, do what you can to shift to sleeping as much as you can at night. Evidence suggests that for those who don’t sleep enough, extending sleep time at night (versus napping) supports better blood-sugar control — and the benefits of doing so can occur in as little as three days.

As a bonus, nighttime sleep also supports the healthy balance of reproductive hormones, such as testosterone, and the ability to better regulate food choices.

Consider the use of melatonin.

Substances that can shift our circadian rhythm are called chronobiotics. The most well-known of these is melatonin, a hormone involved in sleep that is naturally secreted about two hours prior to slumber.

Under normal conditions, when melatonin is released in the evening, it suppresses the nervous system’s firing of the SCN and helps us prepare for overnight rest. However, when we’re exposed to light, our SCN signals our pineal gland to stop the production of melatonin, which is one reason that avoiding light at night is so crucial.

When taken as a supplement in the evening, melatonin can help restore a more normal pattern of your biological clock. The ideal time to take it varies based on the type of melatonin and supplement being used, but I’ve found it can be helpful to trial various ways to time it — just before bed, one hour before bed, two hours before bed — on an evening and subsequent morning where you have a flexible schedule to see what works best for you.

Melatonin might help with more than just sleep, too. One study in postmenopausal women showed that using melatonin for the long term (to the tune of one year) resulted in better body composition due to an increase in lean mass and a loss of body fat.

Another study of individuals with type 2 diabetes who have trouble sleeping  showed that melatonin use was associated with lower levels of hemoglobin A1C, a three-month marker of blood-sugar regulation.

Due to its effect on the circadian rhythm, research on melatonin shows that it often can be successfully used as part of a treatment plan in mood disorders as well.

Manage your caffeine intake.

Caffeine is one of modern society’s most beloved substances — and it may just be the one that can affect circadian rhythm the most. The amount, timing, and frequency of your intake, as well as your genetics (specifically, your ability to clear it), are all crucial factors in determining whether or not to include caffeine in your plan.

Caffeine can not only change your central clock’s regulation, but it may also affect the daily pattern of how melatonin is secreted in your body. Aiming to time caffeine earlier in the day could help with the short-term side effects of circadian rhythm disruption, such as what happens with cross-time zone travel and jetlag.

As a rule of thumb, try to cut caffeine out by noon. If you have contined trouble achieving a regular sleep schedule, consider eliminating it completely for some time to see if that helps.  

Exercise regularly.

Regular exercise has a multitude of health benefits — and regulation of your circadian rhythm is one of them.

In fact, this zeitgeber comes in second place only to light exposure. Some rodent studies even suggest that regular activity can potentially offset some of the adverse effects of light exposure at night. (Note: This in no way suggests that the consequences of staying up late on electronics can be canceled out by grinding out a workout the next day. That can cause undue stress on the system and potentially make matters even worse.) 

Research shows that for those who exercise regularly, both their duration and quality of sleep improves. Consistent exercise can also support evening melatonin levels and help dampen cortisol, a primary stress hormone that has a cyclic rhythm.

Strength training might offer some unique benefits, as the resulting muscle has its own peripheral “mini clock” that is involved in blood-sugar regulation. Aerobic exercise has been shown to support healthy sleep, heart rate, and blood pressure.

There’s some conflicting research around when to exercise. Some studies show that optimal timing likely depends on your chronotype. For example, for resistance training and weightlifting, muscle performance might be better in the morning for early risers, and later in the day for night owls. For aerobic exercise and cardio, it seems that sticking to a consistent time of day is optimal no matter your chronotype.

At the end of the day, consistency is the most crucial aspect — do what you can to maintain a balanced exercise regimen (meaning cardio, strength, and recovery) on a regular basis.

Prioritize socialization.

Our behavior is closely tied to the functions of the SCN and our central clock. Although the SCN is relatively resilient to stressors, chronic social stress and isolation often interrupt our regular physical activity and can negatively influence our eating patterns, both of which can throw off our circadian rhythm. 

Interestingly, studies indicate that the prime time for socializing (measured through patterns of laughing and singing) typically peaks between 8 and 10 a.m., after waking up for the day. This could have some implications for behavior, creative thinking, conversations, and contribution in work meetings and social circles.

Conversely, the increased use of technology in today’s society might be contributing to shifting circadian rhythms and associated social behavior. In an observational study on “friend adding” through virtual platforms, engagement tended to peak in the evening hours between 8 p.m. and midnight — a time when we should be winding down and off our screens.

Adjust your meal timing.

Being mindful of when and how much you eat could have some far-reaching effects on your circadian clock.

Restricting your daily eating window, certain fasting practices (such as intermittent fasting), and methods of calorie restriction all show some promise for improved SCN function — and therefore, improved health.

One pilot study showed that changing the eating window from all day eating (meaning that the first and last foods consumed in a day are spaced more than 14 hours apart) to a shortened 10 to 11 hour window provided positive benefits, especially in the “mini clocks” found in tissues such as the liver and heart. When carried on long term, this way of eating showed it could even reduce certain disease markers.

In animal studies, timing more calorie-dense foods to fall into a narrower daily time slot instead of all day eating (even when the same total calories are consumed) resulted in healthier body weight and improved metabolism. This included a 12 percent reduced body weight, 21 percent reduced cholesterol levels, lower inflammatory markers, more balanced appetite-regulating hormones, and improvements in blood-sugar regulation.

While more research is needed to determine best practices for humans in terms of the “ideal” window of eating for each individual person, there is enough evidence to support the notion that timed feeding windows can help reset the circadian rhythm. Some trial and error can help you determine what works best for you.

When appropriate, practicing a shortened eating window, incorporating habits that provide adequate nutrients while allowing a caloric deficit, and some fasting protocols (under medical supervision) all have promise to support a healthy circadian clock and provide metabolic benefit. Be sure to work with your health care team to determine the best route for you.

Stick to a consistent schedule.

Considering that our circadian rhythm is, well, a rhythm, it makes sense that a predictable day-to-day routine could offer benefits we might not see if we keep an erratic schedule. To the best of your ability, aim to time your sleep, waking, meals, and exercise to be on a relatively consistent pattern.

Most people struggle with staying consistent on the weekends as compared to weekdays. Try to remain mindful of your bedtime and wake time, and consider setting a reminder to help you stay consistent with the timing of your meals. If you go out to dinner, see if an earlier dinner time helps set you up to have an evening schedule that more closely mimics your weekdays.

Wrapping Up

When it comes to optimizing almost any aspect of your health, there’s no doubt that making swaps in the foods you’re eating, adjusting your meal balance of macronutrients (proteins, carbohydrates, and fats), and sticking with an effective strength routine and daily aerobic movement are all crucial.

However, you might be surprised at the many positive benefits that can come from also dialing in the timing and consistency of your day-to-day schedule to get you in a better groove with your circadian rhythm. Improved energy, a healthier body composition, and optimized health and vitality from the inside-out are some of the amazing ways your body is likely to thank you.

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Shift Work and Cancer. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2954516/. Accessed 24 June 2020.

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Samantha McKinney, RD, CPT

Samantha McKinney has been a dietitian, trainer and coach for over 10 years. At first, her interests and experience were in a highly clinical setting in the medical field, which ended up laying a strong foundation for understanding metabolism as her true passion evolved: wellness and prevention. She hasn’t looked back since and has had the honor of supporting Life Time’s members and nutrition programs in various roles since 2011.

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