It is not only what you eat that matters; your body’s biological clock plays a crucial role in how you metabolize food (the biochemical process through which the body converts food into energy).
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My Spanish friend and colleague, Javier Hirschfeld, shared a proverb with me when I asked him about eating habits in Spain: "Eat breakfast like a king, lunch like a prince, and dinner like a pauper"—a saying ingrained in his mind from an early age.
In Spain, it is common for lunch to be the largest meal of the day, while dinners tend to be light. "Some people skip dinner entirely or settle for a piece of fruit and yogurt," he says.
In the UK and the US, however, it is far more common to have a light lunch followed by a larger evening meal.
In doing so, we may have turned things upside down, according to an emerging scientific field known as "chrononutrition." This field suggests that healthy eating is not just about what we eat, but also when we eat it.
From curbing obesity to preventing cardiovascular disease, numerous studies now show that our internal biological clock (the circadian rhythm) plays a powerful role in how our bodies process food and in our overall health. Here is why—and how meals can be structured to take advantage of it.
Marta Garaulet, from the University of Murcia in Spain, first became interested in the impact of meal timing after observations she made at her weight loss clinic.
More than a decade ago, Garaulet noticed two patients who achieved vastly different weight loss results despite living together and following the exact same diet.
One patient had lunch around 1:00 PM, while the other could only eat later due to her lecture schedule. "That was the only difference," says Garaulet.
Meanwhile, the student who ate an earlier lunch lost significantly more weight after six weeks. This prompted her to investigate the intriguing possibility that meal timing itself could have a major impact on weight loss.
In collaboration with colleagues at the University of Murcia, Garaulet tracked 420 adults in Spain participating in a standardized 20-week weight loss program. It turned out that those who ate lunch earlier lost more weight and at a faster rate compared to those who ate later in the day.
Similarly, in a small randomized controlled trial involving 32 healthy women, participants consumed the exact same meals for two weeks. The women who ate lunch later exhibited poorer blood sugar control and burned fewer carbohydrates. Since then, studies have shown that the body's circadian rhythm influences hunger, digestion, and metabolism.
Why Does Timing Matter?
Why does eating the same food at a different time of day have such a profound impact?
When I asked Frank Scheer, a professor of medicine and neuroscientist at Brigham and Women’s Hospital in Boston, Massachusetts, he explained: "Biologically speaking, we are not the same person in the morning as we are in the evening." He notes that our internal body clock regulates nearly all aspects of our physiological functions.
Eating at a time when the body clock indicates it is time to sleep can cause a biological mismatch known as "circadian misalignment."
A prime example is how blood sugar levels are regulated.
In a 2022 study involving over 800 adults, researchers gave each participant a glucose drink designed to mimic the carbohydrate content of a typical evening meal. They found that participants processed the drink far less efficiently when consumed one hour before bedtime compared to four hours before sleep.
Blood sugar levels were higher among those who drank it shortly before sleeping, and they also produced less insulin—the hormone that helps regulate blood sugar levels.
Researchers believe that the hormone melatonin, which rises as the body prepares for sleep, plays a key role in this process.
"If you eat when melatonin levels are high in the evening, your glucose control will be adversely affected," says Scheer, who co-authored the study.
The effect was even pronounced in individuals carrying a common variant of the MTNR1B gene, which is linked to an increased risk of type 2 diabetes. Thus, eating late over time may elevate the risk of developing this disease, which continues to rise globally.
The effects of late eating may extend far beyond blood sugar control. Research indicates that late dining left participants feeling hungrier the following day due to disruptions in hormones that regulate appetite and satiety.
Those who ate late produced less leptin, the hormone that signals to the brain that we are full.
At the same time, meal timing affects how the body stores fat. Rather than breaking down fat, late eating can prompt cells to store it. Over time, this makes it harder for the body to maintain a healthy weight.
Therefore, meal timing may be especially crucial for individuals carrying genetic variants that make them more prone to weight gain.
In a study covering nearly 1,200 adults, an international team of researchers—including Scheer—found that late eating was associated with a higher body mass index (BMI) among individuals genetically predisposed to obesity, whereas eating earlier mitigated this risk.
Cardiovascular Benefits and Night Shift Work
There may also be heart health benefits to eating earlier. A study of more than 100,000 people in France concluded that those who eat breakfast late are more susceptible to cardiovascular disease.
Furthermore, researchers found that extending overnight fasting by eating earlier in the evening rather than delaying breakfast was linked to better heart health.
Researchers suggest this could be particularly important for shift workers and other groups prone to circadian disruption. The advice for night shift workers is to limit food intake during the night and restrict it to daylight hours as much as possible—though this may not be suitable for everyone (healthcare workers, for instance, need to eat to sustain energy levels).
If you feel hungry, stick to small, protein-rich snacks, and try to delay breakfast until the morning if it aligns with your schedule.
Chronotypes and Personal Habits
Naturally, meal timing is not entirely under our conscious control. Individual chronotype—the concept that some of us are early risers while others are night owls—has been shown to influence our eating schedules.
Giovanna Muscogiuri, an associate professor at the University of Naples Federico II in Italy, says morning people are more inclined to follow a Mediterranean diet, whereas night owls tend to delay meals and consume energy-dense foods late at night.
Crucially, chronotype also affects how the body responds to food. Morning individuals reach peak insulin sensitivity earlier, and their bodies secrete melatonin earlier than night owls—who, as a result of late dining, are more vulnerable to impaired glucose control and weight gain.
Muscogiuri advises paying closer attention to our natural tendencies to eat in the morning or evening, adding that late eaters would benefit from gradually shifting their dinners to earlier hours.
We must also consider that sleep duration and quality affect hunger and food cravings. Sleep deprivation makes us more susceptible to overeating and choosing high-calorie foods, which can lead to weight gain over time, according to Muscogiuri.
Practical Advice
The practical advice is relatively straightforward:
Aim to eat at regular times, and consume fiber-rich carbohydrates—such as whole grains, legumes (beans, peas, lentils), and fruit—earlier in the day when the body is better equipped to process them. In the evening, opt for a lighter meal, preferably rich in protein.
In a recent review, Muscogiuri also recommends incorporating sleep-supporting foods into evening meals, such as nuts and seeds, alongside tryptophan-rich protein sources like chicken, salmon, and tuna—as tryptophan has been proven to improve sleep quality.
As for me, I will certainly pay greater attention to my body clock in the future. The Spanish custom of making lunch the largest meal of the day sounds appealing. One thing I might try is cooking in the morning before my workday begins—and perhaps enjoying a plate of paella, too.
So, When Is the Best Time for Dinner?
In truth, there is no single ideal time, but it is best to consume our largest meals earlier in the day and to have our evening meal at least three hours before bedtime.
*Melissa Hogenboom is a senior health reporter for the BBC and the author of "Breadwinners" and "The Motherhood Complex."
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