When should you eat? It’s just as important as what you eat.

We crave food at night—which made sense back when humans only cared about surviving the day. But science shows that to live longer, healthier lives, we need to override our instincts.

By Michael Crupain, Michael Croizen, Ted Spiker
Published 14 Dec 2022, 11:06 GMT
Many of us may crave midnight snacks, but our bodies function better when we eat earlier.
Many of us may crave midnight snacks, but our bodies function better when we eat earlier.
Photograph by Jan Nevidal, Getty Images

Heading into the winter holidays, 'tis the time for eating—and plenty of it. But food can affect your body differently throughout the day, so let’s take a closer look at that internal timepiece.

Understanding your biological clock is the key to shifting your body to optimal function. Here’s how it works: All energy sources originate with the sun. But for times when light is scarce, plants and animals had to develop ways to store energy and reduce energy consumption to survive.

The biological clock is your body’s automated energy conservation system, influencing behaviour on a cellular level from sunset to sunset. It achieves this by sending messages throughout your body via hormones. The signals tell us the best time to sleep and eat on a relatively consistent basis day after day so that you follow the natural cues efficiently. This cycle is your circadian rhythm.

Food and drink affects the human body differently at various times during the day.
Photograph by Gilaxia, Getty Images

Living out of sync

Some people live in opposition to their bodies’ natural instincts—and this is an important example of how food plays a role in the rhythm of life. Take shift workers, who work nights and sleep during the day. Studies examining the health of these populations find that they have increased rates of both sleep issues and obesity. People who work the night shift tend to gain more weight than people with normal nine-to-five schedules.

One study of nurses found that when they switched to a night shift, they actually burned fewer calories than they did when working days, even though they were engaging in the same activities. Other studies determined that shift workers have a 40 percent increased risk of cardiovascular disease as well as heart attacks, strokes, and abnormal heartbeats, compared with those working daytime hours.

Although we don’t completely understand the cause, the main suspect behind the disturbances in the health and metabolism of shift workers is that they are fighting their natural circadian rhythms—and their body’s instinctual notions of when to eat.

Shift workers may be going against their circadian rhythms when it comes to eating, but those who eat an earlier lunch benefit in the long term.
Photograph by Getty Images

Circadian rhythm and your food clock

Our body clock and our food clock have a natural tension point: We crave food at night, but we function better when we eat earlier. Research has shown that in the absence of normal light and time cues, people are naturally the hungriest around the time that would correspond to 8 p.m. and least hungry at the time that would correspond to 8 a.m.

That basic instinct was an advantage in the early days of human existence, but in modern times it may be hurting us. One of the body’s most important hormones for dealing with food is insulin, which regulates the amount of glucose in the blood.

Studies of animals have demonstrated that the body’s secretion of and response to insulin follows a circadian rhythm. Scientists have examined the time of day mammals are the most sensitive and resistant to the effects of insulin; they found that sensitivity is highest during active phases (when they are awake). When a mouse is active, its muscles are moving and it needs energy (in the form of glucose) to perform optimally. Insulin helps move glucose into the muscle cells.

On the other hand, animals are normally the most insulin resistant during their typical sleeping hours. Studies suggest the same phenomena occur in humans. It turns out that mealtime has a big effect on what happens to your blood sugar levels. If you eat the same meal in the morning and at night, your blood sugar will increase more in the evening than in the morning.

Eating the same meal early, like for breakfast, and at night can affect blood sugar levels differently.
Photograph by Getty Images

Fat cells also appear the most insulin sensitive early in the day, with a peak at noon; they are about 50 percent more sensitive midday than they are at midnight. This means that your body is primed to eat at certain times. In fact, eating at the “wrong” time can throw off everything. In a study of mice, those given access to food at the “wrong” time, when they typically sleep, ate more when food first became available—10 percent more total calories during a day—and gained more weight.

Research is starting to show the same effects in humans. One study on weight loss that compared the times of eating among participants over a 20-week period revealed that those who ate lunch earlier lost more weight than those who ate it later. A related study found that those who ate later burned less energy than those who ate earlier.

A gutsy rhythm

Your gut has more to do with your eating habits than just the hungry roar that comes from it. In fact, it’s manipulated by an ecosystem of bacteria—called a microbiome—that influence many, many parts and systems of your body. The bacterial species that inhabit your microbiome can change—and that’s a good thing; people with more diversity of bacteria in their gut seem to be healthier than people with less.

You can influence that diversity by not only what you put in your mouth but also when you eat. In studies of mice, it has been found that many types of gut bacteria populations fluctuate throughout the day on a rhythmic cycle. In one study that analysed mice faeces, researchers found that when the mice were active, they saw more signs of cellular activity that promoted metabolism, cell growth, and repair. When the mice were resting, the researchers found more genes related to activities like detoxification.

Gut health, and by association brain health, is affected by what you eat.
Photograph by Photographs By Getty Images

Earlier is better

Research has shown that our body’s natural rhythm is to want food later, even though it has a negative effect on our overall health. Why are our body’s food cravings out of sync with our circadian rhythm? During periods when we didn’t know when our next meals would come, the human body may have evolved the need for a food-storage mechanism. In that era, humans didn’t live long enough to experience the harms of late-night eating—and in any case, the body only cared about surviving the next day, not the next decade.

Today, we no longer need that extensive storage ability because food is plentiful. We have to consciously override our ancient instincts and make smart choices about when to eat—and that means more in the morning, less later on.

Adapted from What to Eat When: A Strategic Plan to Improve Your Health and Life Through Food, originally published by National Geographic Partners, LLC, on December 31, 2018. Copyright © 2019 Michael F. Roizen and Michael Crupain.

Compilation copyright © 2021 National Geographic Partners, LLC. All rights reserved.

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