Maintaining a healthy weight may depend not only on food choices but also on the timing of meals. A study published in the International Journal of Behavioral Nutrition and Physical Activity found that two habits are associated with a lower body mass index (BMI) in the long term: extending the nighttime fasting period and eating an early breakfast. The study was led by the Barcelona Institute for Global Health (ISGlobal), a center supported by the “la Caixa” Foundation.

Eating Early and Fasting Longer are Associated With a Lower BMI

The findings are based on data from more than 7,000 adults aged 40 to 65 who are part of the GCAT | Genomes for Life cohort, led by the Germans Trias i Pujol Research Institute (IGTP). In 2018, participants provided information via questionnaires about their height, weight, mealtimes, lifestyle habits, and socioeconomic background. Five years later, in 2023, more than 3,000 of these individuals returned for follow-up examinations. The researchers recorded updated measurements and collected new survey data, allowing them to track changes and identify patterns over time.

“Our findings, consistent with other recent studies, suggest that extending the nighttime fast may contribute to maintaining a healthy weight when combined with an early dinner and an early breakfast. We suspect this is because eating earlier in the day is more in sync with the circadian rhythm and allows for better calorie burning and appetite regulation, which can help maintain a healthy weight. However, it is still too early to draw definitive conclusions, so recommendations can only be made once more solid evidence is available,” explains Luciana Pons-Muzzo, who was a researcher at ISGlobal at the time of the study and currently works at IESE Business School.

The Role of the Body’s Internal Clock

The circadian rhythm describes the body’s internal clock, which controls numerous processes throughout the day, including sleep, hormone balance, and metabolism. The body’s ability to process food varies throughout the day: in the morning and early afternoon, insulin sensitivity is higher, meaning the body can absorb sugar from the blood more efficiently and use it as energy. During this phase, calories consumed are burned rather than stored. Toward evening and at night, however, metabolism slows down, insulin sensitivity decreases, and digestion becomes less efficient, causing calories to be stored.

With this in mind, eating early—whether an early dinner or an early breakfast—can help better align food intake with these biological processes. The body receives energy at a time when it can utilize it optimally, which can have a beneficial effect on calorie processing and the regulation of hunger and satiety hormones. At the same time, a longer nighttime fasting period allows the body to enter a state in which it draws more heavily on stored energy reserves and regenerative processes can take place.

The interplay of earlier food intake and longer breaks between meals thus supports the natural alignment between behavior and the body’s internal clock. In contrast, regularly eating late can disrupt this rhythm, which may be associated with less favorable metabolic responses. However, these associations are primarily based on evidence from observational studies to date, so further research is needed to derive clear recommendations.

Gender Differences and Lifestyle Habits

When the researchers compared the results by gender, they found notable differences. Women generally had a lower BMI, adhered more strictly to the Mediterranean diet, and drank alcohol less frequently. At the same time, they reported poorer mental health and were more often responsible for household chores or caring for the family. Using a method called “cluster analysis,” the team grouped participants with similar characteristics. A small group of men stood out. These individuals typically ate their first meal after 2:00 p.m. and fasted for about 17 hours. Compared to others, they smoked more frequently, drank more alcohol, were less physically active, and adhered less strictly to the Mediterranean diet. They also tended to have lower levels of education and higher unemployment rates. The researchers did not observe a similar pattern among women.

Intermittent Fasting and Skipping Breakfast

In intermittent fasting, periods of eating alternate with periods of consciously abstaining from calories. Typically, the body uses energy from food during the eating phase, while it draws on stored reserves during the fasting phase. After a few hours without food, insulin levels drop, and the body begins to increasingly use fat as an energy source. At the same time, processes such as cell repair and metabolic adaptations take place, which are often discussed as potentially beneficial to health.

There are various methods of intermittent fasting. One of the most common is the so-called 16:8 method: you eat within an eight-hour window (e.g., from 10 a.m. to 6 p.m.) and fast for the remaining 16 hours. Another variation is the 12:12 model, which is closer to natural daily rhythms and is often achieved simply by having an early dinner and a late breakfast. In addition, there are models such as 5:2, in which calorie intake is significantly reduced on two days per week.

“There are different ways to practice so-called ‘intermittent fasting,’ and our study focuses on one of them: overnight fasting. What we observed in a subgroup of men who practice intermittent fasting by skipping breakfast is that this practice has no effect on body weight. Other intervention studies with overweight participants have shown that this strategy is no more effective for long-term weight loss than reducing calorie intake,” says Camille Lassale, a researcher at ISGlobal and lead co-author of the study.

Chrononutrition and the Body’s Internal Clock

“Our research is part of an emerging field known as ‘chrononutrition,’ which focuses not only on analyzing what we eat but also on the times of day and the frequency of our meals,” said Anna Palomar-Cros, who was a researcher at ISGlobal at the time of the study and currently works at IDIAP Jordi Gol. “This research is based on the understanding that unusual eating habits can conflict with the circadian system—the system of internal clocks that regulates the cycles of day and night and the associated physiological processes,” she added.

In fact, chrononutrition is a relatively new field of research that links nutrition and time. While traditional nutrition science primarily asks, “What and how much do we eat?”, chrononutrition also asks: “When do we eat—and does that align with our internal clock?” At the center of this is the circadian rhythm, the system of internal clocks that controls nearly all bodily functions. This system ensures that processes such as digestion, hormone production, and energy expenditure are active to varying degrees at specific times of the day. Chrononutrition examines how mealtimes interact with these biological rhythms—or, conversely, come into conflict with them.

A central principle of chrononutrition is that the body is programmed to take in and process food during the day, while repair and regeneration processes take precedence at night. If meals are significantly shifted into the late evening or night hours, this can disrupt this internal balance. This is sometimes referred to as a kind of “metabolic jet lag,” because the body receives signals that do not match the biological time of day. Research highlights several typical factors of influence that are examined in chrononutrition:

• Meal timing: Eating early vs. late affects blood sugar, fat metabolism, and hormone regulation.
• Meal frequency: Many small meals vs. longer breaks between meals can have different effects on metabolism.
• Regularity: A consistent eating schedule appears to be more beneficial than highly variable times.
• Synchronization with light and sleep: Eating late combined with going to bed late can further shift the body’s internal clock.

An example: Two people eat the same meal with identical calories—one at noon, the other late in the evening. Chrononutrition assumes that the body processes this meal differently because hormones like insulin or melatonin act differently at these times. Individual differences also play a role. Some people are more “early risers,” others “night owls.” These so-called chronotypes influence when the body is at its most efficient—and thus also when food is processed more or less effectively. This study builds on previous research by ISGlobal in the field of chrononutrition. Previous findings have shown that eating dinner and breakfast earlier is associated with a lower risk of cardiovascular disease and type 2 diabetes, which supports the idea that the timing of meals plays a significant role in long-term health.