Intermittent fasting (IF) is everywhere these days, a staple of Silicon Valley health “pros” and ketogenic diet fans alike. The eating plan, which has people restrict their eating to specific times of day or days of the week, seems to come with lots of benefits. A 2019 meta-review of studies found that following an IF diet appears to lower blood pressure, cholesterol, and inflammation; help people lose weight; and reduce the risk of developing type 2 diabetes.
But why, exactly, is IF effective? That question has been the subject of debate for years. Some doctors argue that IF is just another form of calorie restriction; by eating for fewer hours during the day, you’re likely eating less overall. Others claim that fasting temporarily triggers a “switch” that supercharges your metabolism. But thanks to new research, experts suggest that IF’s benefits could stem from timing meals to the rise and set of the sun.
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.
Wait, what’s intermittent fasting again? Here’s what you need to know from an RD:
What are circadian rhythms and how do they work?
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, says Steven W. Lockley, PhD, associate professor of medicine at Harvard Medical School and a neuroscientist at the Division of Sleep and Circadian Disorders, Departments of Medicine and Neurology at Brigham and Women’s Hospital. 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.
“If you don’t have that signal, individual cells won’t know what time it is and will drift apart from each other,” says John O’Neill, PhD, a researcher in Cambridge University’s Laboratory of Molecular Biology. This can disrupt normal bodily functions. Think about the jet lag you feel after taking a long flight. That feeling happens because your natural rhythms in your body are out of step with the external signals it’s getting (like, it’s bright light out but you’re normally asleep at this time) and your body is struggling to adjust, making you tired, cranky, and hungry.
How food affects our circadian rhythms
Aside from light, 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 (you know, because you’re sleeping), 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.
What to know before you go out and try fasting for your circadian rhythm
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 are still in the early phases. Many of the above-mentioned 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. (In terms of IF’s general safety, other experts have raised valid concerns on the safety of IF for people who are pregnant or trying to become pregnant, as well as for people with a history of disordered eating.)
However, you likely don’t need to do full-on IF to reap the potential health benefits of a diet timed to your circadian clocks. Instead, follow the long-held advice of health experts and don’t eat for at least two to three hours before you hit the sack. Why? 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.
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