HAMBURG — Make sure to eat breakfast, load up on carbs and fruit in the morning, save your coffee for later, have steak instead of pasta in the evening, and steer clear of midnight snacks: It’s not just about what you eat but when you eat it. That’s the mantra being pushed by influencers and self-styled nutrition gurus across social media and in advice books. The trend is called chrononutrition, eating according to the clock. The promise? You’ll lose weight more easily, sleep better, and maybe even live longer and happier. Sounds pretty good. And tempting, too — maybe you don’t have to give up much, just pay closer attention to the time.
The idea seems to make sense: Our bodies run on an internal clock, the circadian rhythm. It controls when we sleep and wake, and regulates hormones, heart rate, body temperature and metabolism. The thinking goes: If your eating habits are in sync with this rhythm, your body will function at its best. But how much truth is there to that? Is eating by the clock really better for your health?
Let’s start with coffee, the way many people kick off their day. Stop right there, some influencers say, it’s better to hold off for a few hours to avoid crashing later. There’s no doubt that coffee perks you up. Caffeine blocks the receptors in the brain for adenosine, a chemical that builds up during the day and makes you sleepy. Within 45 minutes, caffeine is in your bloodstream and heading for your brain. How long it sticks around varies from person to person. So what is the best time for that first cup?
Cortisol and insulin
A glance at the body clock helps. In the early morning, while we’re still asleep, cortisol levels in the blood rise, peaking shortly after we wake up. “The rising cortisol is your body’s signal to get going,” says biochemist Henrik Oster, who studies chronophysiology at the University of Lübeck. To avoid messing with this natural rhythm, chronobiologists suggest drinking coffee earlier rather than later.
There’s another good reason to drink it in the morning: Coffee may help lower the risk of heart disease, diabetes and allergies. And according to a study involving more than 40,000 participants, those benefits are greater when the coffee is consumed in the morning rather than throughout the day.
After coffee comes — often nothing. Nearly half of people skip breakfast on weekdays, rushing off to work or skipping it due to intermittent fasting. British biochemist Terence Kealey recommended exactly that years ago. In his book Breakfast Is a Dangerous Meal, he even argued that breakfast is bad for your health.
Now, influencers like German web video producer Pamela Reif and French biochemist and author Jessie Inchauspé, known as the “Glucose Goddess,” argue for a solid, nutrient-rich breakfast instead. It’s better aligned with your metabolism, they say, and makes it easier to lose weight. It’s not far from what Grandma always said: Eat breakfast like a king, and dinner like a pauper.
It’s not far from what Grandma always said: eat breakfast like a king, and dinner like a pauper.
In fact, clocks throughout the body regulate digestion depending on the time of day. During daylight hours, the pancreas produces more insulin to lower blood sugar. The intestines produce more enzymes to aid digestion. And the liver, fat tissue and muscles all process sugars, fats and proteins more efficiently in the morning. So maybe Grandma was onto something. But what does science say about breakfast’s impact on health?
The most controversial meal of the day
Breakfast is the most studied meal of the day. To test how meal timing affects health, researchers ideally split participants into groups and give them meals at different times — a setup similar to a lab experiment. But these studies are expensive and difficult to conduct over long periods. More often, participants are simply instructed how to eat in daily life. This is tricky to monitor: birthdays, brunches and barbecues inevitably get in the way. And nutrition studies in general come with caveats. They can only show correlations. They usually can’t prove that diet alone is the cause of better health. People with unhealthy diets are more likely to smoke and exercise less. And people with less money often can’t afford fresh produce or whole grains.
Overall, those who ate breakfast had more stable insulin levels
For a review of breakfast studies, researchers from Australia and the UK analyzed nine studies, only one of which was conducted in a controlled lab setting. Overall, those who ate breakfast had more stable insulin levels, healthier blood sugar, cholesterol and fat profiles, and lost slightly more weight than those who only had coffee for breakfast. But compared to people who ate more in the evening, the difference wasn’t huge: On average, breakfast eaters lost just over a kilogram more.
A 2022 study pointed in a similar direction. One group of participants ate breakfast for several days, while another skipped it and had a big dinner instead. Both groups consumed the same amount of carbohydrates, fats and proteins, and had similar routines for exercise and sleep. The results: The breakfast-skippers burned less energy, and their fat metabolism changed in a way that encouraged fat storage. They also felt hungrier during the day. Unfortunately, the researchers didn’t track whether this actually led to weight gain.
Dinner is the main meal in many countries
That increased hunger from skipping breakfast showed up again in another study from the UK. All the participants were overweight and wanted to slim down. Those who ate breakfast said they had better control over their hunger, and that food wasn’t constantly on their minds. They also had fewer cravings for sweets and fatty snacks. Meanwhile, those who only had a small morning snack and a big dinner felt hungrier throughout the day. But again, the scale didn’t budge: Both groups burned the same amount of calories and lost the same amount of weight. That may be because the study only lasted four weeks, and both groups consumed the same number of calories — something the researchers closely monitored.
But in real life, nobody’s counting your calories for you. Maybe that’s where breakfast really shines: It keeps you feeling full longer. That could help reduce overall food intake, curb snacking, and make it easier to maintain a healthy weight.
It’s a social thing, too.
Still, in many Western countries, dinner remains the main meal of the day. It’s a social thing, too. Families eat together in the evening, friends gather over snacks and wine, and dates happen over dinner. Plus, cravings for sweet or salty treats tend to kick in at night.
But the body’s internal clock tells a different story. As the sun sets, melatonin levels rise, signaling that it’s time to rest. If you eat a rich dinner late at night, your pancreas still has to churn out insulin. So while your brain is winding down, your digestive system is kicking into gear. Scientists call this “internal desynchronization” — like your body is experiencing jet lag from within.
The human body is far too complex
Researchers believe this could explain why people who consume half their daily calories after 8 p.m. are more likely to be seriously overweight and have more body fat — regardless of how much they eat or exercise. Late-night eating also raises the risk of type 2 diabetes, high blood pressure and heart disease. This is especially true if you shuffle off to the fridge during a TV binge and help yourself to a snack.
“Of course you can pull an all-nighter or eat late and not get sick,” says biochemist Henrik Oster. Even during Ramadan, when people eat only at night, the effects on metabolism are short-term. Once the fasting period ends, things return to normal. A healthy body can handle some short-term disruption to its rhythm.
But shift workers with chronically irregular hours often have trouble sleeping. They also tend to have higher body mass indexes, elevated cholesterol and blood pressure. They’re more prone to chronic conditions like cardiovascular disease and obesity. But is that because they’re out of sync with their body clock — or simply because they aren’t sleeping enough? People who don’t sleep well are often hungrier and eat more. The jury is still out.
People respond differently to the same foods.
So what can we really say about the best time to eat? The answer is simple but not straightforward: It varies from person to person. People respond differently to the same foods. Some see quick spikes in blood sugar and insulin; in others, fat lingers longer in the blood. Even identical twins show these differences.
That’s why nutritionist Martin Smollich of the University Hospital Schleswig-Holstein is wary of chrononutrition hype. “These trends are great for selling books, diet plans and supplements,” he says. While there’s no doubt our biological systems follow a daily rhythm, “we can’t derive hard-and-fast nutrition advice from that. The human body is far too complex.”
At the end of the day, your body clock isn’t the only thing that matters. People in Southern Europe tend to eat late, yet the Mediterranean diet is still considered one of the healthiest. Those who follow it are less likely to develop cancer or die of heart disease. As chronobiologist Henrik Oster puts it: “What we eat still has a bigger impact on our weight than when we eat.” In a world where food is available 24/7 and eating patterns are all over the place, that’s actually good news. Our bodies are more resilient than we think.
