Midnight snacks, second breakfasts, late-night bites, midmorning nosh. As a culture, we eat around the clock, incorporating food into social gatherings, work hours and even downtime.
Case in point: One study conducted by the Salk Institute for Biological Studies in La Jolla, California, asked participants to use a mobile app to track everything they ingested over the course of a day. More than half the participants reported eating for a span of 15 hours or longer — almost from the time they woke up until when they turned in for the night.
But is all-day dining a problem? If you stick to healthy, whole foods and keep an eye on your overall caloric intake, is there really any difference between eating three squares before 7 p.m. and nibbling on a dozen mini-meals right up to bedtime? As it turns out, when you eat is just as important as what you eat. Thought the explanation is hundreds of thousands of years old, scientists are just beginning to understand it, thanks to recent research on circadian rhythms and the body’s internal clock.
The Body Clock
The phenomenon known as the circadian rhythm is based on the rotation of the earth. As part of our species’ evolution, our bodies have adapted to daylight and nighttime, regulating certain biological functions accordingly — sleep, metabolism and hormone production, to name a few. In 2017, three American scientists won the Nobel Prize in Physiology or Medicine for their discovery that all multicellular organisms — humans included — have so-called “clock” genes that dictate wake and sleep patterns. They found that these genes still continue to function even when the organism (a fruit fly, in this case) was kept in complete darkness, proving that our internal clock doesn’t just react to light and day — it actually keeps time along with it. And that internal timekeeper is very picky about when we should and should not be eating.
Timing Is Everything
Pamela M. Peeke, M.D., MPH, assistant professor of medicine at the University of Maryland and author of The Hunger Fix: The Three-Stage Detox and Recovery Plan for Overeating and Food Addiction (Rodale, 2012), explains that our natural eating patterns are part of our “primal software.” Cave women, she reasons, ate early in the day when they woke up hungry, and food was readily available and easiest to procure. Nighttime was for sleeping and hiding from predators.
Even though we no longer have to worry about T. rex and modern refrigeration keeps our favorite eats within arm’s reach 24/7, we’re better off sticking to our ancestors’ more restricted feeding schedule for a variety of reasons.
First, certain times of day are more conducive to processing certain macronutrients — aka carbohydrates, proteins and fats. While healthy fats like avocado, nuts and olive oil can be eaten throughout the day, Peeke suggests consuming the majority of your carbohydrates (bread, grains, fruits, starchy vegetables, etc.) in the morning and early afternoon. “Up until about 3 p.m., you’ve got different insulin secretion and therefore have a more optimal glucose-burning engine going at that time,” Peeke explains. “It becomes less and less efficient after 3 p.m.” Lunch should be your biggest meal of the day, and for dinner, your focus should be on lean protein, healthy fat and lighter, water-based carbs like broccoli and green beans.
Second, your body requires periods of nonfeeding to perform a host of reparative functions. Satchin Panda, Ph.D., a professor at the Salk Institute and author of The Circadian Code (Rodale, 2018), explains that about six to eight hours after your last meal, your body switches from burning available sugar and glycogen stores to burning fat. Besides producing ketones, which have been shown to improve brain function, this fat-burning phase runs parallel with an important detoxifying process called autophagy. “The body will go around and see whether there’s any recyclable material available and recycle it,” Panda explains. By sweeping up damaged proteins and organelles (structures within a cell), autophagy helps counteract disease and aging.
Your gut (specifically your gut lining and microbiome) also requires a break from digestion, since it works all day breaking food into fuel. According to Panda, the gut generates a new lining every 10 to 15 days: Every night your body removes 7 to 10 percent of the old cells and replaces them with new cells. “You cannot repair a road when the traffic is still moving,” Panda says. “That’s why you cannot repair gut lining properly if you just ate and went to sleep.”
Graze No More
So how long should you refrain from eating in order to get the optimum benefits? Research suggests at least 12 to 14 hours. The same study that revealed our propensity for all-day grazing eventually introduced its participants to “eating windows.” Those who typically ate over the course of 14 hours or longer were asked to restrict their eating to between 10 and 12 hours. They were allowed to choose their own time frame and were not required to alter their caloric intake or change the quality of their food. After 16 weeks, participants reported multiple benefits and expressed an interest in continuing with time-restricted eating. According to Panda, who co-wrote the study, participants experienced higher energy levels and deeper sleep.
“Deep sleep is very important because only [then] do we produce growth hormone, and that repairs our gut lining, our skin and some muscle,” he says. Participants also lost an average of 7.2 pounds, most of which they kept off for one year after the study.
Of course, one could argue that by limiting the number of hours during which you can eat, you may naturally cut calories (and dodge less-than-nutritious late-night temptations). However, another study conducted by Panda and his team compared the health outcomes of two groups of mice — one with unrestricted access to food and another with access to the same exact amount of calories but only over the course of 10 hours. Even though they consumed the same diet, the mice with free rein became obese and developed metabolic diseases while the mice who ate within a 10-hour window remained healthy.
The take-away? Even a diet filled exclusively with nutritious foods can work against your health and fitness goals if it’s not timed properly. Mind your body’s clock, feed it properly and you’ll soon be in sync.
Restriction ≠ Deprivation
In practice, eating according to your circadian rhythm isn’t particularly difficult. With a little extra planning and few adjustments to your current schedule, you can reap the benefits of time-restricted eating without feeling deprived. Here are a few basic guidelines to get you started.
Plan your macronutrients accordingly. Consume the majority of your carbs before 3 p.m. when your body most efficiently burns glucose. In the late afternoon and early evening, focus on protein, cruciferous, and leafy vegetables and healthy fats.
Take note of the time when you take your first bite of the day (yes, coffee with milk and/or sweetener counts), then time your last meal so you finish eating 12 hours or less later. For example, if you grab a breakfast wrap at 7 a.m., finish eating dinner by 7 p.m.
If you eat your last meal a bit later than you’d like, Pamela M. Peeke suggests delaying your breakfast the following morning to allow for an adequate nonfeeding period. While you don’t always have control over dinner (no one’s suggesting you force the early-bird special on all your friends), you usually have the final say on when you eat breakfast.