A study conducted at Brigham and Women’s Hospital (part of Mass General Brigham) found that the body burns fewer calories when digesting food in the evening, and the reason lies in the internal biological clock rather than in sleep or physical activity.
A sandwich eaten at 7 p.m. requires less energy for the body to digest than the same sandwich eaten at 7 a.m. This result comes from a small, strictly controlled experiment conducted at Brigham and Women’s Hospital and published in the journal Metabolism.
This timing effect is unrelated to sleep, sitting still, or sitting in the dark; researchers eliminated all these factors. What remained was simply the internal 24-hour clock operating independently. Consuming nearly half of one’s daily calories after 5 p.m. leads to higher blood glucose levels, even when the food and calorie count remain the same.
This study examines another aspect of the issue: how much energy the body expends digesting food and how that changes throughout the day.
Eating raises the metabolic rate for several hours. The intestines must process the ingested food, while the liver and muscles must either store or burn it. All of this requires energy.
Researchers refer to this additional calorie burning as diet-induced thermogenesis, or DIT. Most textbooks estimate it at approximately 10 percent of a person’s daily caloric intake.
Researchers have known since the early 1990s that DIT levels are higher in the morning than in the evening. A 1993 study involving nine young men who ate the same meal at 9 a.m., 5 p.m., and 1 a.m. showed that the morning meal triggered the greatest surge in activity.
No one could pinpoint the exact reason. The people eating in the morning had simply been asleep; then dawn broke, and they had gone 10 or 12 hours without food. Any of these factors could have explained the difference on its own.
A team of researchers led by Han-Chow Ko and Frank Scheer from the Medical Chronobiology Program at Brigham and Women’s Hospital wanted to determine whether the body clock alone could reproduce this rhythm.
“It is unclear whether this is due to the influence of our biological clock,” the researchers wrote in a Q&A published by Brigham and Women’s Hospital.
To answer this question, they used a method circadian biologists call a “constant routine.” The study involved sixteen overweight or obese adults—12 men and 4 women—with an average age of 36. They remained awake for 36 consecutive hours in dim light, reclining in bed, without exercising or looking at a clock.
Every six hours, they ate an identical small meal. After each meal, the team measured oxygen consumption and carbon dioxide exhalation, allowing researchers to calculate the number of calories burned per minute.
Since every meal, posture, light level, and interval between meals was identical, any fluctuations in DIT levels over the 36-hour period could only originate from within the body.
This study was part of a larger clinical trial registered on ClinicalTrials.gov and funded by the National Institutes of Health. A distinct rhythm was evident. Calorie consumption peaked during the biological morning—around 8 a.m. for these participants—and hit a low around 8 p.m. The fluctuation between the peak and the trough was 44 percent, corresponding to roughly 10 calories over the four hours following a meal.
Resting metabolism also rises and falls in sync with the biological clock, even in the absence of food intake. Consequently, the team re-analyzed the data after subtracting this background rhythm.
After this adjustment, DIT still exhibited a circadian rhythm, peaking around 7 a.m. and bottoming out around 7 p.m. The fluctuation narrowed to 29 percent, equivalent to about seven calories per meal.
“Our biological clocks cause diet-induced thermogenesis to peak in the biological morning,” the researchers wrote.
In a preliminary check, participants who had eaten early in the days prior showed the same rhythm as those who had eaten late. It appears the rhythm did not shift based on recent habits—at least not during the course of the experiment.
Seven to ten calories per meal is a small amount; it is roughly equivalent to a third of an average apple. The study did not measure changes in body weight, and the authors do not claim that this, in itself …explains weight gain associated with late-night eating. They describe this rhythm as a mechanism that “may contribute to weight gain or difficulties in losing weight.”
Scheer’s laboratory had previously investigated the effects of late-night eating from a different angle. In a 2022 study published in the journal Cell Metabolism, volunteers consumed the same meals according to two different schedules, with one shifted approximately four hours later than the other.
According to a press release, those following the late eating schedule burned fewer calories throughout the day. The study also noted that participants on the late schedule experienced greater feelings of hunger.
Findings regarding glucose levels came from a separate group of scientists at Columbia University and the Open University of Catalonia. They explained that insulin secretion decreases at night. The Massachusetts General Hospital study adds another factor: the metabolic cost of digestion itself, which is also influenced by evening changes in blood sugar levels.
Previous research has linked earlier dinners to slower organ aging, while another study associated later breakfasts in older adults with a slightly increased risk of mortality. The American Heart Association has also warned that irregular meal times can disrupt heart rhythms.
The sample size of sixteen people was small, and three-quarters of the participants were men. All fell within the overweight or obese BMI range, with an average BMI of 28.8. It remains an open question whether the same fluctuations in calorie expenditure occur in lean adults, the elderly, or night-shift workers. On an ordinary day, no one spends 36 consecutive waking hours in a dimly lit room. The controlled protocol was designed to eliminate the influence of real-life circumstances; thus, the figure of seven calories reflects the body clock’s activity in isolation from reality.
In everyday life—where sleep, physical activity, and daylight come into play—the effect could be stronger. Or it could be weaker.
The study resolved a question that had remained open since 1993: the morning advantage in calorie burning persists even in the absence of sleep, indicating that at least part of this effect is driven by the body’s daily rhythm rather than by specific morning activities.
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