In life, timing is everything. We’re ruled by the clocks on our collective wrists, walls and smart devices. We count minutes on treadmills and then calories afterward. We race to business meetings, doctor’s appointments, trains and dinner dates. Time-starved, we somehow manage to crowbar in a quick power walk or a brief call with a friend. Sitting down to eat becomes mission impossible in our category 5 “hurry-cane” of mindless grabbing and going, dashboard dining, stuffing our face on the job, skipping meals, guzzling gallons of sugary caffeine, and nighttime binging.

And sleep, the only respite from this chaos, becomes an afterthought, an option grossly minimized and often omitted. Then, the morning alarm rings, we stumble out of bed and start the cycle all over again—unaware that we’re paying for this pandemonium in body dollars. Girths expand, energy wanes, and blood sugars rise, launching a predictable, desperate rush to the latest diet or exercise fad.

All the while, we convince ourselves it’s the acceptable new normal to blast through the day, casting healthy lifestyle habits aside in the endless pursuit of social and professional goals.

We chase external clocks, oblivious to the most critical lifesaving timepiece—our body clock. Few seem to realize that our very survival depends on living in sync with this internal timing system.

We’re ignoring body time, unaware how far we’ve strayed from the most natural, powerful rhythm of life. But we can get back in sync. It’s just a matter of understanding the body clock and adapting our nutrition to its natural rhythms.

Origins of the Body Clock

Our internal timing device is a network of 37 trillion cellular clocks coordinated by a master clock—the suprachiasmatic nuclei (SCN)—located in the brain’s hypothalamus. The SCN’s 20,000 neurons control the body’s daily rhythms, including sleep, physical activity, attentiveness, hormone regulation, body temperature, immune function and digestion (Inouye & Kawamura 1979).

This web of body clocks is integral to our survival. For most of our 2-million-year evolution, our ancestors lived by the diurnal light/dark rhythms of the Earth’s rotation. Human metabolism, behavior, physiology and biochemistry have always been intimately linked with this daily planetary cycle.

Basically, we evolved to live in sync with the Earth, internalizing the planet’s natural timing into a 24-hour pattern known as the circadian rhythm. Circa is Latin for “around,” and diem means “day.” Driven by body clocks, cir­cadian rhythms are endogenous (orig­inating within an organism’s cells) and entrainable (adjusting to the local environment). They synchronize with seasonal and tidal rhythms (Edery 2000).

How the Body Clock Works

For many years, the workings of our internal circadian biological clocks remained a mystery. Several decades ago, chronobiologists began closely studying intrinsic body clocks and extrinsic factors that could affect their function. These extrinsic factors are called zeitgebers (German for “time cues”) and include light, temperature and time zones (Sharma & Chandrashekaran 2005). With the advent of new genetic-testing technology, researchers finally pieced together the mechanisms governing the self-sustaining clockwork and intricate feedback loops inside each cell.

In 1994, Joseph Takahashi and colleagues identified the clock gene, which plays a major role in activating elements integral to the generation of circadian rhythms (Vitaterna et al. 1994). Also in the 1990s, Jeffrey Hall, Michael Rosbash and Michael Young set to work pinpointing genes linked to regulating the body clock network. In subsequent years, they worked methodically to elucidate the inner workings of the biological clock.

Their collective discovery of the period, timeless and doubletime genes added critical pieces to the body clock puzzle and helped explain, for instance, how light synchronizes with the clock (Hardin, Hall & Rosbash 1990; Vosshall et al. 1994; Price et al. 1998). As a result, we now recognize that biological clocks function by the same principles in cells of other multicellular organisms, furthering our understanding of how plants, animals and humans adapt their biological rhythms to the Earth’s rotations.

These discoveries were so groundbreaking that the three scientists received the 2017 Nobel Prize in Physiology or Medicine (Nobel Assembly 2017). And circadian biology has evolved into a highly dynamic research field where researchers now implicate body clock function in health and well-being.

It All Works Together

In an interview with The Scientist magazine after the Nobel Prize announcement, Young said:

“We didn’t realize at the time that this clock would be represented all over the body in many different tissues and control so many different biological processes that we go through every day. It left us with the understanding . . . that our whole bodies are rhythmically active.

“I think that the biggest lack of understanding right now is that we really are rhythmic, biologically, and that those rhythms are very important to the smooth working of the human body.”

Young cautioned that skipping meals and skimping on sleep disrupts the body’s natural, coherent rhythmicity. “From experiments ongoing in animals, all of these things produce deficits in health,” he said (Taylor 2017).

Thus, our well-being suffers when there is a mismatch between our lifestyle habits and our inner timekeeper. Getting out of sync with our biological clocks and rhythms risks a rise in obesity, diabetes, heart disease and cancer.

The body clock genes play a critical role in metabolic function. Chaotic eating, relentless stress, continuous screen-gazing, sleep deprivation and a sedentary lifestyle disrupt our powerful circadian rhythm—wreaking life-threatening havoc on our mental and physical health.

At the extreme, what happens when humans reverse the typical light/dark pattern—for example, by working the night shift? Researchers have documented that this kind of schedule increases the risk of metabolic disorders (obesity, diabetes) and can result in a 40% increase in the risk of cardiovascular disease (Bøggild & Knutsson 1999). Metabolism is primed by the circadian machinery encoded in our genes, and disruption can have dire health consequences.

Getting Our Rhythm Back

The solution is clear: We need to get back in sync with our natural circadian rhythm. But how?

First, through our mouths! Is it time for you to eat yet? Read your body clock! Pay attention to your natural circadian rhythm. Learning to nourish yourself guided by your body clock network sets off a domino effect that can correct and reset sleep and physical activity routines and rituals, along with metabolism.

Nutrition and fitness professionals expend much time and effort focusing on the quality and quantity of food consumed. There’s no question that switching out processed foods for whole, organic foods is key to good nutrition. So is paying attention to how much one eats.

But we don’t place enough emphasis on the frequency and circadian timing of meals. From an evolutionary perspective, contemporary society’s most common pattern of eating—three meals with snacks interspersed throughout the day—makes no sense.

How It All Started

Step back and think about this. Up until 12,500 years ago, when humans invented agriculture, finding food was an unpredictable and tricky proposition. Awakened by the first rays of sunlight and hungry from not eating for about 12 hours, our predecessors quickly “assumed the vertical” and began to hunt, gather and forage for whatever food was available.

If our ancestors were fortunate, they had one main meal, possibly two, per day. And if they encountered a patch of plump strawberries, rest assured they didn’t run away crying out, “Too many carbs!” Anything and everything was in play. Energy consumption and expenditure were in sync with the requirements of daily biological and environmental rhythms. Our innate primal survival software was honed on periodic feast and famine, entrained by the opportunities and limitations of our living environment (Mattson et al. 2014).

Even when agriculture made food more secure and abundant, the agrarian lifestyle maintained a natural light/dark rhythm for thousands of years. After all, Thomas Edison didn’t invent the first commercially practical incandescent light until 1879 (Matulka & Wood 2013). Without house lighting, smartphones, tablets, computers and TVs running 24/7, it was early to bed and early to rise, accompanied by farming chores galore and a 2- to 3-mile walk to school.

For some people, this powerful metabolic drive is alive and well today—even if they wish it weren’t. Musician Moby noted: “I’m envious of people who can sleep as long as they want. I have the circadian rhythm of a farmer” (Alvarez 2010). Overweight and obesity were not a major issue until farming became mechanized and people left the fields to work in cities and towns.

Finding the Key to Dietary Rhythms

Happily, we now have an army of researchers crafting a revolutionary new model for improving our health by getting back into sync with our circadian rhythm. Leading the pack is Satchidananda Panda, PhD, of the Salk Institute, who believes that the rise of eating later at night may have thrown off the circadian rhythm that evolved in humans, possibly contributing significantly to increases in obesity.

In 2012, Panda published a milestone study (Hatori et al. 2012) comparing two groups of mice fed a poor-quality, high-fat diet (akin to the typical American diet). Both groups received the same number of calories. One group ate whenever they wanted in 24 hours, while the other ate only during an 8-hour window in sync with a mouse’s circadian rhythm. The mice were free to make typical physical movements but were not exercised.

The results were remarkable. The mice that ate without time restrictions became overweight and diabetic. The time-restricted feeding group gained little weight and developed no metabolic problems. Further, compared with the unchecked eaters, the time-restricted eating group showed no signs of obesity, hyperinsulinemia (high insulin levels in the blood), hepatic steatosis (fatty liver disease) or inflammation. And in addition to having optimal body composition, their motor coordination improved.

It appears that timing of food consumption is vital in protecting against disease and preventing accumulation of excess body fat.

In a longer follow-up study by Panda and his team (Chaix et al. 2014), mice eating at all hours throughout a 38-week experiment became obese and metabolically ill. Those time-restricted to a 9- to 12-hour window remained lean and healthy, even if, during the study, they were allowed “weekend cheats” (i.e., could eat outside their typical time).

Here’s the clincher: Halfway through the study, the unrestrained eaters were switched over to a 9- to 12-hour window. During that time, they began to shed the extra weight they’d gained via freestyle eating. Panda and his colleagues proved that obesity was reversible using time-restricted feeding. Could this regimen potentially be a nonpharmacological strategy to use against obesity and metabolic disorders?

<>p>In an observational study published in 2015, Panda followed 156 adults using a free research app he created called myCircadianClock. His team found that more than 50% ate over the course of 15 hours and only 10% restricted meals and snacking to the more ideal 12 hours or less. The majority were out of sync with their circadian rhythms (Gill & Panda 2015).

Research Supports Circadian Eating

Emerging findings from a host of animal and human studies now suggest that honoring the primal circadian rhythm and restricting feeding more to the daylight hours within the natural daily light/dark pattern can improve health and prevent disease. This knowledge has spawned an industry devoted to “intermittent fasting,” defined as a diet that concentrates on the “when” of eating, not the “what” or “how much” (Collier 2013).

Time-restricted feeding is a way to apply the theory of circadian rhythm. Academics have studied a variety of related methods, and some people have taken the basic concepts of published research and commercialized them. The more popular plans include fasting sporadically for 24 hours, Krista Varady’s alternate-day fasting (Varady et al. 2013), 2-day-per-week fasting, fasting for up to 16 hours and the 5-days-per-month “fasting-mimicking plan” popularized by Valter Longo (Brandhorst et al. 2015). Because the term “fasting” carries a variety of cultural and religious connotations, we’ll use “nonfeeding,” to avoid misperceptions.

Which Method Works Best?

Once we accept the idea of letting our circadian rhythms dictate our feeding times, it’s logical to wonder which method of feeding/nonfeeding works best. To answer that question, we need to recall our beginnings. The precariousness of the environment, seasonal changes and the unpredictability of the food supply forced our ancient ancestors to constantly adjust their feeding and nonfeeding times. Survival, after all, was a dynamic process, and the human body’s cells and genes were built with the flexibility to meet the challenge. Therefore, humans most likely used a multitude of feeding strategies as daily challenges forced them to adapt.

Thus, no one perfect solution emerges as a guide for us today. But the common thread is that, on average (and by necessity), early humans had at least 12 hours of nonfeeding. If we adopt that pattern, is something special and beneficial happening during those hours of nonfeeding? Indeed, it is.

Balance and Variety

During circadian-based feeding hours, the goal is to consume appropriate portions of a varied balance of whole foods with high-quality sources of protein, fat and carbohydrate—individualized to a person’s specific needs. There’s no “perfect” nutrition plan that accommodates everyone’s distinct age, gender and medical requirements. Instead, there’s a broad spectrum of healthy, nutritious options we can try with great success.

Another nuance to eating by our natural body clock is that consumption happens when the body is more efficient at breaking down foods. The metabolic system evolved to make fuel and energy resources available at specific times of the day. Carbohydrates are more optimally metabolized in the morning and early afternoon, based on insulin levels and sensitivity (Qian & Scheer 2016). Cholesterol is broken down in the body by bile acids, which are controlled by an enzyme most present upon awakening (Gnocchi et al. 2015). Random, chaotic eating patterns degrade the liver’s ability to switch glucose production on and off. This leads to higher blood sugar levels and eventually to diabetes.

The Nighttime Switch

Once daylight fades into nighttime and feeding ends, we enter a unique alternative metabolic phase that relies less on glucose for fuel and more on ketone-body-like carbon sources. After approximately 6 hours of nonfeeding, stored glucose, or glycogen, is depleted enough to initiate a shift from glucose metabolism to fat metabolism. During this ketone-dependent phase, significant beneficial metabolic events are initiated by the liver through changes in gene expression and multitudes of complex enzymatic pathways (Longo & Panda 2016). Since this process typically happens in the evening, the person using this diet is actually sleeping through most of these regenerative and restorative hours.

What Happens During Nonfeeding Hours

When people enter a nonfeeding state, a robust process of regeneration and repair begins at the cellular level, often peaking at about 12 hours food-free. The removal of old and dysfunctional cellular debris and toxins is called autophagy, from the Greek auto (“self”) and phagein (“to eat”).

Cells recycle their contents to promote optimal health and prevent disease. This cellular housekeeping is so critical to life that the 2016 Nobel Prize in Physiology or Medicine went to Yoshinori Ohsumi, PhD, who identified the genes essential for autophagy (Sedwick 2012). This discovery led to a new paradigm in our understanding of how cells degrade and recycle and why this is so important for health and wellness. For instance, cells use autophagy to eliminate damaged proteins and organelles, counteracting negative consequences of aging.

Mark Mattson, PhD, senior in­ves­tigator at the Laboratory of Neuro­science at the National Institutes of Health, found that 12–16 hours of nonfeeding reduces excess body fat while lowering levels of insulin and blood sugar. He also found that it’s healthful for the body to impose the mild cellular stress of 12–16 hours of nonfeeding,
as it strengthens cells’ ability to adapt and cope with more severe stresses like disease and aging. Moreover, Mattson has shown that nonfeeding protects against stroke and slows cognitive decline, such as that seen in Alzheimer’s disease (Longo & Mattson 2014; Sykora et al. 2015).

Valter Longo, director of the University of Southern California Longevity Institute, created a unique plan called the fasting-mimicking diet, which deliberately induces cellular stress with low-calorie, high-fat, low-carb, low-protein nutrition for 5 days in 1 month. This periodic time-restricted feeding had similar effects in animals and humans: optimized multisystem regeneration and muscle maintenance, decreased visceral fat, and enhanced cognitive performance and life span. Longo argues that a 5-day intensive nutritional challenge can produce sustainable beneficial gains in critical biomarkers for several months afterward (Choi et al. 2016).

Large Gains From Small Efforts

Small changes can yield significant benefits. Shaving off even a few hours from the typical feeding time makes sustainable weight reduction possible. Panda’s group published a study in which eight healthy but moderately overweight people (BMI 25–30) adjusted their feeding time from their typical 14–15 hours to 10 hours. They ate their normal foods and didn’t change their exercise habits. Over the course of 16 weeks, they not only shed an average of 4% of their body weight but also reported significantly improved energy levels and more satisfying sleep. At the 1-year follow-up, most were still adhering to the 10-hour feeding time (Gill & Panda 2015).

Research shows that Americans tend to consume up to two-thirds of their daily caloric intake after 3 p.m. Binge eating also tends to be a nocturnal phenomenon (Longo & Panda 2016). Planning healthy nutrition around body clocks driven by the circadian rhythm can shake up these self-destructive habits. If you follow the natural light/dark cycle, then you’ll be feeding for 8–12 hours during daylight and nonfeeding, or regenerating, for 12–16 hours, including 8 hours of sleep. The key is to eat sometime during daylight, as late-night or all-night eating interferes with glycogen depletion, ketone-body generation, cellular autophagy and optimal metabolic resets.

Practically speaking, the easiest and most basic plan is simply to feed within a 12-hour window, ideally in sync with the light/dark cycle, say 7 a.m. to 7 p.m. This is reasonable and immediately cuts out those excessive nocturnal calories. One of the top three weight management companies, Jenny Craig®, has become the first major industry leader to successfully incorporate 12 hours of feeding and 12 hours of regeneration (the Rapid Results™ plan) into its consumer offerings.

Increasing the nonfeeding time to anywhere between 12 and 16 hours simply enhances the health benefits of the regenerative period. In essence, you’re going primal, heeding age-old survival genes. And there’s plenty of wiggle room. Remember, no 2 days are alike. Eating challenges abound, and hours shift. All you have to do is strive to hit that minimal 12-hour nonfeeding time window, even if you’re not perfectly in sync with light and dark. It’s okay. Cave men and women had plenty of daily variability as well.

In sum, the health benefits of a time-restricted feeding pattern reflect the dynamic interplay of its two unique elements—feeding in accordance with the light/dark circadian rhythm and regenerating during nonfeeding hours. Surely, that’s a powerful and very welcome physiological win-win.




Alvarez, L. 2010. He’s sensitive about the pancakes. New York Times. Accessed Apr. 16, 2018:

Bøggild, H., & Knutsson, A. 1999. Shift work, risk factors and cardiovascular disease. Scandinavian Journal of Work, Environment and Health, 25 (2), 85–99.

Brandhorst, S., et al. 2015. A periodic diet that mimics fasting promotes multi-system regeneration, enhanced cognitive performance, and healthspan. Cell Metabolism, 22 (1), 86–99.

Chaix, A., et al. 2014. Time-restricted feeding is a preventative and therapeutic intervention against diverse nutritional challenges. Cell Metabolism, 20 (6), 991–1005.

Cheng, A., et al. 2012. Involvement of PGC-1╬▒ in the formation and maintenance of neuronal dendritic spines. Nature Communications, 3, 1250.

Cheng, A., et al. 2016. Mitochondrial SIRT3 mediates adaptive responses of neurons to exercise, and metabolic and excitatory challenges. Cell Metabolism, 23 (1), 128–42.

Cheng, C-W., et al. 2017. Fasting-mimicking diet promotes Ngn3-driven ╬▓-cell regeneration to reverse diabetes. Cell, 168 (5), 775–88.

Choi, I.Y., et al. 2016. A diet mimicking fasting promotes regeneration and reduces autoimmunity and multiple sclerosis symptoms. Cell Reports, 15 (10), 2136–46.

Collier, R. 2013. Intermittent fasting: The science of going without. CMAJ, 185 (9), E363–64.

Edery, I. 2000. Circadian rhythms in a nutshell. Physiological Genomics, 3 (2), 59–74.

Fann, D.Y., et al. 2014. Intermittent fasting attenuates inflammasome activity in ischemic stroke. Experimental Neurology, 257, 114–19.

Gill, S., & Panda, S. 2015. A smartphone app reveals erratic diurnal eating patterns in humans that can be modulated for health benefits. Cell Metabolism, 22 (5), 789–98.

Gnocchi, D., et al. 2015. Lipids around the clock: Focus on circadian rhythms and lipid metabolism.Biology, 4 (1), 104–32.

Hardin, P.E., Hall, J.C., & Rosbash, M. 1990. Feedback of the Drosophila period gene product on circadian cycling of its messenger RNA levels. Nature, 343, 536–40.

Hatori, M., et al. 2012. Time-restricted feeding without reducing caloric intake prevents metabolic diseases in mice fed a high-fat diet. Cell Metabolism, 15 (6), 848–60.

Inouye, S.T., & Kawamura, H. 1979. Persistence of circadian rhythmicity in a mammalian hypothalamic “island” containing the suprachiasmatic nucleus. Proceedings of the National Academy of Sciences, 76 (11), 5962–66.

Longo, V.D., & Mattson, M.P. 2014. Fasting: Molecular mechanisms and clinical applications. Cell Metabolism, 19 (2), 181–92.

Longo, V.D., & Panda, S. 2016. Fasting, circadian rhythms, and time-restricted feeding in healthy lifespan.Cell Metabolism, 23 (6), 1048–59.

Marosi, K., & Mattson, M.P. 2014. BDNF mediates adaptive brain and body responses to energetic challenges.Trends Endocrinology and Metabolism, 25 (2), 89–98.

Mattson, M.P., et al. 2014. Meal frequency and timing in health and disease. Proceedings of the National Academy of Sciences, 111 (47), 16647–53.

Matulka, B., & Wood, D. 2013. The history of the light bulb. Accessed Apr. 16, 2018:

Nobel Assembly at Karolinska Institutet. 2017. The Nobel Prize in Physiology or Medicine 2017, Jeffrey C. Hall, Michael Rosbash, Michael W. Young. Accessed Apr.16, 2018:

Price, J.L., et al. 1998. Double-time is a novel Drosophila clock gene that regulates PERIOD protein accumulation.Cell, 94 (1), 83–95.

Qian, J., & Scheer, F.A. 2016. Circadian system and glucose metabolism: Implications for physiology and disease. Trends in Endocrinology & Metabolism, 27 (5), 282–93.

Sedwick, C. 2012. Yoshinori Ohsumi: Autophagy from beginning to end. Journal of Cell Biology, 197 (2), 164–65.

Sharma, V.K., & Chandrashekaran, M.K. 2005. Zeitgebers (time cues) for biological clocks. Current Science, 89 (7), 1136–46.

Sykora, P., et al. 2015. DNA polymerase ╬▓ deficiency leads to neurodegeneration and exacerbates Alzheimer disease phenotypes. Nucleic Acids Research, 43 (2), 943–59.

Taylor, A.P. 2017. Q&A with Michael Young, Nobel Laureate. The Scientist. Accessed Apr. 16, 2018:–Nobel-Laureate/.

Varady, K.A., et al. 2013. Alternate day fasting for weight loss in normal weight and overweight subjects: A randomized controlled trial. Nutrition Journal, 12 (1), 146.

Vasconcelos, A.R., et al. 2014. Intermittent fasting attenuates lipopolysaccharide-induced neuroinflammation and memory impairment. Journal of Neuroinflammation, 11(85).

Vitaterna, M.H., et al. 1994. Mutagenesis and mapping of a mouse gene, clock, essential for circadian behavior. Science, 264 (5159), 719–25.

Vollmers, C., et al. 2009. Time of feeding and the intrinsic circadian clock drive rhythms in hepatic gene expression. Proceedings of the National Academy of Science, 106 (50), 21453–58.

Vosshall, L.B., et al. 1994. Block in nuclear localization of period protein by a second clock mutation, timeless. Science, 263 (5153), 1606–09.

Zarrinpar, A., Chaix, A., & Panda, S. 2016. Daily eating patterns and their impact on health and disease. Trends in Endocrinology & Metabolism, 27 (2), 69–83.

Pamela Peeke, MD, MPH, FACP

Leave a Comment

When you buy something using the retail links in our content, we may earn a small commission. IDEA Health and Fitness Association does not accept money for editorial reviews. Read more about our Terms & Conditions and our Privacy Policy.


Related Articles

Is It Time to Eat Yet?

Science finds that circadian rhythms give excellent signals on the right times to feed ourselves.

Subscribe to Our Newsletter

Stay up to date with our latest news and products.