In many aspects of life, we all know that when matters as much as what. A marriage proposal can be a wonderful thing—but not in the middle of a fight about dried toothpaste in the sink. Asking for a raise is a necessary part of work life—but not right after you spilled coffee on a client. Cheering for your favorite team is one of life’s joys—but not if you’re checking the score on your phone during your kid’s school concert.
The same applies to how you should think about food as it relates to your overall health, longevity, and weight: When matters.
That principle, of course, is the entire foundation of this book. You absolutely need to think about what you’re eating, which is what we’ll cover in Chapter 2. But a number of other factors dictate how food functions. Therefore, the ideal approach is combining what you eat with when you eat it.
Your body is a dynamic ecosystem of organs, tissues, genes, and chemicals that change from day to day and during the course of each day. That means you cannot just treat it like a vehicle that requires the same gas and oil to run smoothly in every situation. To best fuel your biological ecosystem, you need to think about the nuances of timing, not just the dangers of Alfredo sauce. The tricky part is that timing happens on two different levels, both equally important.
On one hand, you have to consider your body’s specific circumstances. Your body ebbs and flows through various emotions, hormone levels, and health situations, meaning that any number of environmental stresses affect the way your insides are working. Because food adds another variable to these scenarios, what you eat during these periods plays a role in how your body reacts. Your ecosystem changes as your fuel sources do. In Part 3, you will encounter a huge swath of scenarios—some emotional (what to eat when you’re hangry), some medical (what to eat when you want to prevent heart disease), and some situational (what to eat when you cannot sleep). All have different answers, because your body needs different nutritional solutions as it manages the various stresses, insults, and dynamics of life.
On the other hand, when also refers to the time of day during a normal 24-hour cycle. This is all about how your body reacts to food and how food affects your body throughout the rhythms of a day—and this is what the root of this book is all about, nudging you to switch your eating habits to optimize how food works to improve your body.
Let’s take a closer look at this clock—and how food works as you chug, churn, and chew throughout the day.
CIRCADIAN RHYTHM 101
Many diet and eating plans focus on the best kinds of food to eat—for good reason, because that notion is really at the heart of smart eating. Some plans also address the element of timing, perhaps advocating several small meals throughout the day, while others might assert that the when doesn’t matter at all: It’s all about calorie counting and portion control.
This is one of the many reasons why eating well can be more confusing than a 225-box Sudoku. Although you won’t get much argument about some health-related things (few would disagree that processed sponge cakes aren’t as healthy as snap peas), the area of when to eat is hotly debated and not well understood.
But we can end that confusion here, by looking deep into a part of your biology that you are rarely thinking about when it comes to eating: your biological clock, also known as your circadian rhythm. Sure, we talk about it when it comes to sleeping, and we throw around the phrase when talking about fertility. But we rarely discuss the biological clock in the context of jalapeño omelets.
This, however, is the key to shifting your body to optimal function: Align your eating with these natural rhythms, and you’ll have food working better with your body, rather than against it.
Here’s how it works. All energy sources originate with the sun. But because the sun isn’t always present, plants and animals had to develop mechanisms for storing energy and reducing energy consumption when light was scarce. So life on Earth developed automated or instinctual processes to conserve energy, which improved the chance for survival. No sun, no energy. No energy, no life.
Now, you may consider instincts as things you just do without conscious thought, like nurturing a baby or hightailing it in the opposite direction of a saber-toothed cat. But you also have instincts that lurk much deeper in your biology: organ-level or cellular-level reflexes that you don’t necessarily notice, but that are just as important. The biological clock—your body’s automated system for conserving energy—is one of them, as it influences behavior from sunset to sunset.
It achieves this by sending messages throughout your body via hormones. The signals—a biological Batman signal, if you will—tell us when to sleep and when to eat. Over and over and over. This cycle is your circadian rhythm. These natural cues—whether for animals or plants—are relatively consistent from day to day, so that your biological clock dictates a rhythm that you follow effectively and efficiently.
Your body wants to work as efficiently as possible, and these instincts help ensure it doesn’t waste time or valuable resources. The internal clock conserves your body’s reserves, allowing you to live in this rhythm every day. (Modern-day humans seem to have developed an internal clock requiring 7 a.m. Starbucks stops.)
In mammals, the master biological clock that sets the circadian rhythm is located in the brain. It’s a tiny area of just 20,000 brain cells in the hypothalamus called the suprachiasmatic nucleus (you have around 86 billion neurons in the brain total).1,2 This clock keeps time in a near 24-hour cycle, requiring light input that travels to it from the eyes and allows it to keep its rhythm set at exactly 24 hours.3 The clock sends chemical signals out to the rest of the body that prime it to engage in certain activities, based on the time of day.
You probably understand this phenomenon mostly by the way it works in regard to sleep. Although you may interpret getting tired at certain times of day as a signal to tuck in and snooze, you may not realize that other things are also happening as a response to your biological clock. For example, your body temperature changes throughout the day based on your circadian rhythm. At night, it decreases, and melatonin rises, which encourages you to sleep. In the morning, it rises along with corticosteroid hormones, encouraging you to wake up and take on the day.4 Your big brain gives you the ability to override these biological cues and stay up all night if you want. But we all know the price we pay when we don’t live in tune with our natural body clock.
Now, some people choose to live their lives in opposition to their body’s natural instincts—and this is an important example of how food plays a role in the rhythm of life. About 15 million Americans have jobs as shift workers, where they work during the night and sleep during the day.5 Studies examining the health of these populations find that they have increased rates of both sleep issues and obesity.6 People who work the night shift tend to gain more weight than people with a normal nine-to-five schedule.7 One study of nurses found that when they switched to a night shift, they actually burned fewer calories than they did when working the day shift, even though they were engaging in the same activities.8 Other studies determined that shift workers have a 40 percent increased risk of cardiovascular disease as well as heart attacks, strokes, and abnormal heartbeats than those working daytime hours.9
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.
CIRCADIAN RHYTHM AND YOUR FOOD CLOCK
You may not know it, but you feel the effects of your clock every day. You may pretty consistently feel hungry or sleepy or energetic at the same times from day to day. Maybe you nap for 10 minutes when you hit the couch at 7:30 p.m., or maybe you reach for something to nosh on at three or four in the afternoon. If you’re a pet owner, you’ve probably observed this in dogs; those that are fed at the same time each day change their patterns in anticipating food. Scientists examining these displays found that animals indeed have individual food clocks,10 an instinct that ensures we get the right amount of food throughout the day so that we have plenty of energy for survival.
Humans seem to have a food clock, too—and this is the area that few of us have really tapped in our daily habits. This is what serves as the scientific backbone of What to Eat When: Sync your body clock with your food clock.
What makes this tricky is that our bodies have a natural tension point: We crave food at night, but we function better when we eat earlier. Let’s see how this works.
In an amazing 2013 study, researchers at Brigham and Women’s Hospital in Boston put volunteers into a lab to measure how changes in light govern our circadian rhythm. Normal periodic light cues were removed and replaced with constant artificial light, so that researchers could see what the natural cycle of the body’s clock was. Researchers fed the brave participants, who lived in the lab for 13 days, at different times of day throughout the study and asked them to rate their hunger. (No word on whether TV networks wanted to turn this into a reality show.)
They found that in the absence of normal light and time cues, people are naturally most hungry around the time that would correspond to 8 p.m., and least hungry at the time that would correspond to 8 a.m.11 That basic instinct was an advantage in the early days of man—but in modern times, it may actually be hurting us (more on this in a moment).
Another interesting note: Your organs—including the liver, heart, muscle, pancreas, fat, lungs, and kidneys—seem to have their own food clocks, too.12 That is, they all can develop “preferences” about when you eat, depending on their functions.
What sets the clock? The suprachiasmatic nucleus, as it happens. But these organs have an intrinsic rhythm, meaning they’re trying to run based on what’s best for your body.
One of the body’s most important hormones for dealing with food is called insulin. You’ve probably heard of it, because its levels are abnormal in people who suffer from diabetes. In fact, diabetes starts when muscle and fat cells become resistant to insulin—meaning that even though insulin might be there, it doesn’t have the effect it is supposed to (we’ll talk about insulin a lot more in the following chapter). 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 or resistant to the effects of insulin; they found that sensitivity is the 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 glucose into the muscle cells.
On the other hand, animals are normally the most insulin resistant during their typical sleeping hours (this is also the time they have the highest levels of fasting blood sugar).13 During the rest phase, the mouse is typically inactive and food is not coming in. But whereas muscles don’t need as much energy, the rest of the body does not shut down during sleep. In fact, this is prime time for crucial processes like DNA repair. One theory as to why we have higher insulin resistance at night: Your brain needs energy while you sleep because it is actually doing work during this down period (like storing memories and getting rid of waste). But because your muscles do not, their insulin resistance may allow more glucose to go to the brain during this period (the brain is not insulin resistant, so it can receive the extra circulating glucose).
What It Means for You: Studies of humans suggest that these same phenomena occur in you. Your body is most sensitive to insulin in the morning and becomes more resistant as the day goes on.14,15 Now you’ve probably heard that eating a carbohydrate-rich meal causes a rise in blood sugar, and the more carbohydrates you eat, the higher this rise will be. At least that’s what nutritional scientists thought. It turns out that the time of day you eat that meal 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 actually increase more in the evening than in the morning.16,17 Fat cells also appear to be 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.18
This means that your body is primed to eat at certain times and that the “what to eat when” principle really does matter. In fact, it matters so much that eating at the wrong time can throw off everything. Scientists at the University of Alabama at Birmingham have studied what happens to mice when they eat at the “wrong” time. Researchers gave mice access to food either during the night only, when they typically eat, or during the day only, when they are typically sleeping. The rest of the time, they had no food available.
Compared with those on a more normal schedule, mice that only had food available at the wrong time ate more when food first became available—10 percent more total calories a day—and gained more weight. In addition, mice fed during the inactive phase tended to use carbohydrates more for energy rather than burning fat, compared with mice fed during the active phase.19
Tinkering With Timing: These same researchers then conducted another series of experiments. They gave the mice either a high-fat or low-fat diet early or late. This time, they found that mice given a high-fat meal at the end of the day, rather than at the beginning of the day, had more insulin resistance, ate more calories, and gained more weight.
When mice ate more closely to when they were going to sleep, their bodies were not able to compensate for the extra calories by increasing energy expenditure. But the researchers weren’t satisfied. They wanted to know what would happen if the two groups ate the same number of calories in this scenario. So they decided to give one group a high-fat meal at the beginning of the day and one at the end; both groups received the same number of calories. This time, they found that those in the group that ate at the end of the day weighed more—and had more body fat and increased insulin resistance.20 So it looks like when the mice eat is just as important as what they eat. Their bodies are primed for food during the active phase—and the earlier they eat, the better.
Research is starting to show these 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.21 And a related study found that those who ate later burned less energy than those who ate earlier.22
The take-home in all of this: It’s better for your body to eat earlier. We need to shift our habits to front-load, rather than back-load, our daily eating rituals.
THE MICROBIOME’S CIRCADIAN 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 that influences many, many parts and systems of your body.
The bacteria that make up your gut are called your microbiome. The bacterial species that inhabit your microbiome can change—and that’s a good thing, because people with more diversity of bacteria in their gut seem to be healthier than people with less. You can influence that diversity not only by 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. This means that the relative abundance of particular bacteria change on a rhythmic cycle. One group of scientists from Israel tested mouse feces to try to find out what might be going on. They collected the mouse poop (dirty job, but someone’s gotta do it) and analyzed it for bacterial genes.
What did they find? (No, not digested cheese.) When mice were active, they found 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.23 In mice that have had their circadian clock systems knocked out, the normal microbiome schedule disappears. This is because these mice lose normal feeding behaviors. Controlling when these mice eat can restore a more normal pattern to the fluctuations of the gut microbiome that happen throughout the day—once again showing the importance of when you eat.24
The Israeli group went on to study how eating at the wrong time could affect health. They shifted the light-dark cycle that the mice saw in the lab eight hours at a time to simulate international jet lag. As you would expect, the microbiomes in the jet-lagged mice were different than in the normal mice. When the scientists fed a high-fat diet to the jet-lagged and the normal groups, only the jet-lagged mice gained weight and became glucose intolerant.
To determine whether the composition of the microbiome caused this circumstance, they killed the gut bacteria with antibiotics. The antibiotic-treated jet-lagged mice did not gain weight or have glucose intolerance, suggesting that the microbiome was playing a significant role. Next, the researchers performed a fecal transplant from the jet-lagged mice to mice with no gut bacteria; after the transplant, the mice had an increase in body fat.25
So what’s the takeaway here? Eating early is generally better for your microbiome, which is better for you. And the best time to eat whole grain carbohydrates, protein, and dietary fat is early as well.
EAT EARLY
Here’s why the when part of the eating equation is so tough: 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.
That’s the major conflict: Our bodies were designed to want more calories at the end of the day and fewer in the morning. But the optimal way of eating—from a circadian rhythm point of view—is to consume more energy earlier in the day and less energy later in the day.
Why are our bodies’ food cravings out of sync with our circadian rhythm? During times when we didn’t know when our next meals would be coming, 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, that impulse didn’t make much of a difference: The body only cared about surviving the next day, not the next decade. Today, we no longer need that extensive storage ability; our modern world has outpaced the human body’s ability to adapt to its new environment where food is plentiful.
We have to override our reptilian instincts using the executive function of the brain to make smart choices about, well, what to eat when.
If we can take one lesson from the existence of our biological clock, it’s that our bodies will work best when we stay in sync. So remember this mantra: More in the morning and less later on.
Now, it’s important for you to understand some food fundamentals. In the next chapter, we will explore the what of food and how it works, so you can combine it with the when.