The Connection Between Circadian Rhythms and Overall Health

Explore this article:

⋅ Your Internal Clocks
⋅ How to Get the Best Rest

1. Sleep Timing
2. Food Timing
3. Light Exposure

⋅ The Rise of Circadian Medicine
⋅ Circadian Rhythms and Gene Expression

If you’ve ever traveled across time zones, you’ve likely experienced what chronobiologists call “circadian disruption.”

Jet lag is the most acute example. Your brain grasps what the clock on the wall shows, but your body does not. That’s because various organs in your body contain “peripheral clocks” set to their own circadian rhythms, and they become confused by long-distance air travel: Your pancreas thinks it’s still in London, your liver is stalled over Greenland, and your kidneys suspect you’re back home in Akron.

You’re hungry for a turkey dinner in the middle of the morning, nodding off at 4 in the afternoon, and wide awake at 3 a.m., with no hope of sleep.

Still, you don’t need to fly around the world to experience out-of-sync body clocks. A disruption occurs anytime we get too little or poor-quality sleep or when we get most of our sleep during the day. Misalignment may even be provoked by slight day-to-day shifts in bedtimes and wake times, like when we hang out with friends late on the weekend and sleep in the next day — a phenomenon dubbed “social jet lag.”

The effect of circadian rhythms on our well-being is substantial — and not only because of their role in helping us fall asleep and wake up. When our internal clocks get out of sync, our bodies do too.

“Circadian health is important because it is closely linked to both physical and mental health,” says Phyllis C. Zee, MD, PhD, a professor of neurology at Northwestern University who specializes in sleep and circadian medicine. “There’s a preponderance of evidence that when your circadian rhythms are misaligned, it can be a risk factor for chronic conditions like diabetes, heart disease, obesity, and cancer.”

Chronic circadian disruption has also been linked to poor concentration and memory, diminished performance, increased risk-taking behavior, problematic detoxification, and mood and anxiety disorders — as well as a range of other acute and chronic health concerns. (See “Circadian Rhythms and Gene Expression” at the end of this article.)

Shift work, for example, can create a perfect storm of circadian-disrupting factors: exposure to light at night when our bodies are primed for sleeping, decreased melatonin production, and nighttime eating. Rotating shift work, during which sleep patterns continually change, is even more disruptive. Research showing a link between shift work and increased breast-cancer risk is so well established that nighttime shift work has been dubbed a potential carcinogen.

A study published in Sleep in 2022 found that older adults exposed to any amount of light at night (including a blinking light on an electronic device or a streetlight outside the window) have an increased risk for obesity and diabetes. Even one night of sleep with ambient light exposure increases insulin resistance the next morning.

Unsurprisingly, the key to circadian alignment is adequate sleep. Yet quantity is only part of the equation: Sleep quality matters, as do sleep timing, food timing, and light exposure.

That’s why many healthcare practitioners now prescribe a host of lifestyle strategies — rather than just another pill — to help restore good sleep habits and align the body’s rhythms.

Your Internal Clocks

The body’s master clock is in the brain — specifically, in a group of cells in the hypothalamus called the suprachiasmatic nucleus, or SCN. Zee describes the SCN as the “circadian pacemaker.” It’s located just above the optic nerve and responds to 24-hour cycles of light and dark, using them to help orchestrate the body’s many rhythmic functions. This part of the brain also regulates hormones, including cortisol, insulin, and melatonin, as well as glucose for energy.

The peripheral clocks in our organs — the pancreas, liver, kidneys, heart, gastrointestinal (GI) tract, muscle tissue, adipose tissue, and breast tissue — respond to instructions from the SCN involving a range of functions, including hormone production and digestion.

In short, it’s not just sleeping and waking that are cyclical. Our entire body operates according to rhythmic schedules. “These rhythms are generated at a cellular level in single cells,” Zee explains. “Every cell in your body contains the molecular machinery to generate these 24-hour rhythms.”

Our entire body operates according to rhythmic schedules. “These rhythms are generated at a cellular level in single cells,” Zee explains. “Every cell in your body contains the molecular machinery to generate these 24-hour rhythms.”

The system works like an orchestra, with the SCN as the conductor and the peripheral clocks as the musicians. Each instrument can be played at its own tempo, but if musicians did this during a performance, the orchestra would sound cacophonous. To make harmonious music, each follows the conductor and stays on beat.

When our peripheral clocks are synchronized, that orchestra is well tuned — a state of circadian alignment known as entrainment. When we’re entrained, Zee says, we’re better able to predict our best times for eating, ­activity, and performance. Our sleep and wake times naturally become more regular, and we feel better overall.

When the peripheral clocks and SCN are out of sync, however, confusion results — often in the form of mixed physiological messages. For example, if it’s 11 p.m. and your SCN sends out signals to prep for bed, but your GI tract believes it’s noon and tells you it’s time for lunch, you’ll have a harder time falling asleep because you’re hungry and thinking about sandwiches. “Ideally, your metabolic systems are ready to eat when you’re awake, whereas they’re turning all that down when you’re sleeping,” says Zee.

If the cyclical secretion of melatonin or cortisol (triggered by signals from the hypothalamus, where the SCN is housed) gets out of whack, we may get conflicting physiological messages about when to be active and when to rest. A strong urge to nap during the day or lying awake at 2 a.m. are both signs of circadian disruption. (For more on cortisol’s effects, see “How to Balance Your Cortisol Levels Naturally.”)

The most common causes of circadian disharmony are ill-timed or insufficient light exposure, as well as irregular sleep: too much, too little, or too late in the morning. “The SCN is very sensitive to light,” Zee explains.

This disharmony soon becomes a self-perpetuating cycle, she adds. We need regular exposure to natural light and dark because they are “the strongest synchronizing agents” for the circadian system.

Bright light at night is particularly disruptive to our circadian rhythms. The SCN requires stimulation from daylight to stay on track and coordinate internal circadian rhythms with the 24-hour physical environment.

“The best entraining solution is exposure to light during the day and having the unopposed ability through darkness to elevate your natural rise in melatonin at night,” says Zee.

“The best entraining solution is exposure to light during the day and having the unopposed ability through darkness to elevate your natural rise in melatonin at night,” says Zee.

Darkness allows the pineal gland to produce melatonin. Exposure to bright light at night disrupts the production of this soothing hormone and makes it harder to fall asleep.

Likewise, bright light stimulates the SCN through the optic nerve, which prompts your body’s clocks to assume their daytime roles. Zee notes that even some people who are blind may not be “circadian blind”; their SCN is still able to respond to bright light.

“The core message here is that you need to have enough, appropriately timed, and sufficient sleep as well as light exposure,” she says.

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How to Get the Best Rest

To align your internal clocks and ensure deep, restorative sleep, pay attention to these three key factors.

1) Sleep Timing

Keeping consistent sleep and wake times is one of the best ways to synchronize your body clocks, says integrative-medicine physician Aviva Romm, MD. “A lot of people are very dysregulated in both. Having them tightly controlled is really important.”

This means sticking to the same sleep hours every day of the week — even on weekends. Deviating about half an hour on either side is fine. But that’s it, no matter how tempting it is to stay up until 3 a.m. and sleep until Sunday brunch.

A consistent wake time may be the most critical ­element for circadian health, says functional-­medicine practitioner José Colón, MD, MPH, author of The Sleep Diet: A Novel Approach to Insomnia. And he believes it should be on the early side.

“I see it all the time with my patients,” he says. “I ask about when in their lives they got the best sleep and they will say, ‘When I was in the military’ or ‘When I was at summer camp.’ It’s that early wake-up time.”

Colón advises waking by 7 a.m. An early-rising routine helps ensure that melatonin will naturally begin to ramp up around 8 or 9 p.m. This helps you grow sleepier in time to drift off by 11 p.m.

If you’re unable to maintain consistent sleep and wake times and feel tired during the day, Colón suggests avoiding lengthy midday naps — anything more than 40 minutes is probably too long. Extended naps prematurely purge a chemical called adenosine that accumulates throughout the day and induces drowsiness in the evenings. So, come bedtime, the sleepy feeling never arrives.

Shorter naps, however, can boost your daytime vitality without disrupting your nighttime sleep, says Colón. Catnaps are still OK.

2) Food Timing

Going to bed on a reasonably empty stomach can also help with sleep. Zee recommends concluding your evening meal two to three hours or more before bedtime. Because late hours and low light trigger the SCN to signal the peripheral clock in the GI tract to shut down metabolic activity, the body is less equipped to break down food at night.

This is one reason night-shift work can lead to health complications. “People are eating at night, which makes sense, but their bodies never completely adjust,” says Daniel Buysse, MD, a University of Pittsburgh professor of sleep medicine.

Even if someone is accustomed to being awake at night, the digestive and metabolic systems perceive this as an “adverse circadian time,” so night-shift meals are harder for the body to process. “That in itself may contribute to the weight gain and obesity that is often associated with night-shift work,” he explains.

Zee recommends concluding your evening meal two to three hours or more before bedtime. Because late hours and low light trigger the SCN to signal the peripheral clock in the GI tract to shut down metabolic activity, the body is less equipped to break down food at night.

When we eat late, we tend to make more indulgent food choices. Social psychology researchers have posited that willpower is a limited resource that diminishes throughout the day. This is one reason a cupcake may have a mystical power over us after dark — especially if we’re tired, stressed, and trying to work late into the evening.

“No one craves a salad at 11 p.m.,” acknow­ledges Zee. “We know that if you’re sleep-deprived, you’re more likely to want to eat carbs and make unhealthy food choices.”

Alcohol right before bed can also disrupt sleep. A nightcap may seem like a natural way to wind down, but sleep experts recommend against it. While drinking may reduce the time it takes to fall asleep, it tends to ­delay the onset of REM sleep — which studies have shown can translate into less restful sleep overall.

If you’re going to enjoy a cocktail, limit it to one drink with dinner at least three hours before bed. This will give your body time to metabolize the alcohol well before lights out.

3) Light Exposure

Exposure to light and dark is a critical — and often overlooked — component of circadian health. Bright natural or natural-mimicking light during the day helps regulate melatonin production.

You don’t need to be exposed for long periods of time, says Zee: A morning stroll with your dog, a bike commute to work, eating lunch outside, or even making sure your office space is brightly lit can help.

Sunglasses protect your eyes, but Colón warns that wearing them constantly can reduce melatonin production. (If your eyes are never exposed to bright light, the onset of darkness won’t trigger the pineal gland to produce the hormone.) Forgo your shades in the morning light to reset your sleep–wake cycle.

Forgo your [sunglasses] in the morning light to reset your sleep–wake cycle.

Receiving bright light in the morning is another reason an early wake-up time is so important, he adds. That’s when blue-wavelength light, the most stimulating type, is strongest. It has the most powerful effect on the circadian cycle because the receptors in our retinas are especially sensitive to it, and they transmit these stronger signals to the SCN. (In the late afternoon, we see more red- and orange-wavelength light, which are less stimulating.)

Blue light after dark can be a surprisingly powerful sleep disrupter. “One of the biggest things we know that affects sleep is using any kind of electronics ­before bedtime,” says Romm. Blue light is produced by computer, phone, and tablet screens, as well as by energy-efficient light bulbs.

When we’re checking email on our phones and reading on tablets as we nod off, we’re at odds with our natural rhythms. “The circadian system evolved expecting that our days would be more or less filled with light and activity, and our nights would be filled with darkness and much less activity,” says Buysse.

Romm recommends powering down all screens in the evenings at least an hour before bed. If that’s not feasible, try a setting or an app that dims blue light on smartphones, tablets, and computers, such as the Night Shift setting for Apple or the Dimmer app for Android.

Keep lamps on their lowest setting after dinner, and consider getting a red light bulb or pink salt lamp for your bedroom. Not only is the aesthetic romantic, but our brains associate red-spectrum light with the waning of sunlight and the end of the active day.

If possible, keep lamps on their lowest setting after dinner, and consider getting a red light bulb or pink salt lamp for your bedroom. Not only is the aesthetic romantic, but our brains associate red-spectrum light with the waning of sunlight and the end of the active day.

The rhythms our body clocks keep are a foundational part of our physiology. When we attune to them and try to align our schedules a little more closely with the rising and setting of the sun, our health will benefit — around the clock.

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The Rise of Circadian Medicine

Until 2014, most circadian researchers believed our body clocks regulated between 10 and 30 percent of our genes. They’ve since discovered that closer to 50 percent of our genes follow a 24-hour cycle, and this revelation has sparked a surge of research — some of which examines whether circadian rhythms might be leveraged to create more-effective medical care.

Because circadian-controlled genes follow a predictable daily schedule, healthcare practitioners have begun to experiment by timing medical interventions for maximum impact. Just as you’d drink coffee or tea in the morning to help yourself wake up and avoid them in the evening when your body needs to wind down, the best time to target circadian-controlled genes with medication may be the point in a 24-hour period when they would be most receptive to its effects.

Findings from a review published in the journal Hypertension in 2021 suggest that taking blood pressure medications at a specific time each day may improve outcomes.

Emerging research on circadian rhythms and cancer has also produced eye-opening results. A study published in 2022 found that circadian dysregu­lation accelerates tumor growth in young people with colorectal cancer. Eating at night, sleeping during the day, and getting too little sleep are all risk factors for this population, but emphasizing a rhythmic schedule for food and sleep may help improve their health outcomes and boost survival rates.

Even tumors have a circadian schedule.

Even tumors have a circadian schedule. Metastatic tumors spread by producing and circulating tumor cells throughout the body, and experts have long assumed they shed these invasive cells constantly. But a study published in Nature in 2022 tells a different story: Metastatic breast tumors do not produce invasive cells constantly; instead, they create the most cells when the body’s asleep and are relatively unproductive during waking hours.

What’s more, the cells produced at night are likely to metastasize, whereas cells made during the day lack the ability to morph and spread. The study authors write that these findings provide “a new rationale for time-controlled . . . treatment of metastasis-prone cancers.”

Research into circadian-informed medical interventions is still in its infancy — and barriers abound: Studying large groups of individuals who have slightly different circadian schedules or who have tumors with different cyclical rhythms is a challenge; convincing research subjects to take medications within an hour-or-so window is tricky; and resistance from big pharmaceutical companies is real.

Still, the promise of circadian medicine has a lot of experts feeling optimistic. They believe it’s a healthcare approach whose time has come.

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Circadian Rhythms and Gene Expression

Studies suggest that roughly half of our genes are expressed rhythmically.

“The most robustly rhythmic genes that are being expressed are metabolic genes and genes of the immune system — for example, those that regulate inflammation,” says Northwestern University sleep specialist Phyllis C. Zee, MD, PhD. This may explain why disrupted sleep has such a strong association with increased risk for chronic diseases like type 2 diabetes, cancer, and Alzheimer’s.

Type 2 diabetes is a metabolic issue; cancer nearly always involves increased inflammation in some form; and ­Alzheimer’s is now widely understood as an inflammatory condition. Research also suggests this latter disease is partly attributable to an inability of the brain to detoxify, a process that takes place during deep sleep.

“This connection with chronic disease and metabolic disorders is a really good example of that interaction, where these rhythmic profiles of gene expression can be modified by light and by sleep states,” says Zee.

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This article originally appeared as “Get In Sync” in the June 2023 issue of Experience Life.