Does eating late at night make you fat?

Does eating at night make you fat? Now there is evidence to support the weight loss benefits of eating at the right time of day — and trying to avoid the others.

food inside the fridge at night

Does eating at night make you fat?

There may be a good reason to add one more “must” to your weight-loss rules: eat at the right time of day.

A 2009 Northwestern University study found that eating at irregular times — the equivalent of the middle of the night for humans, when the body wants to sleep — influences weight gain. The study is the first causal evidence linking meal timing and increased weight gain, and suggests that the regulation of energy by the body’s circadian rhythms may play a significant role.

Our circadian clock, or biological timing system, governs our daily cycles of feeding, activity and sleep, with respect to external dark and light cycles. Several medical studies have found the body’s internal clock also regulates energy use, suggesting the timing of meals may matter in the balance between caloric intake and expenditure.

“How or why a person gains weight is very complicated, but it clearly is not just calories in and calories out,” said Fred Turek, professor of neurobiology and physiology in the Weinberg College of Arts and Sciences and director of the Center for Sleep and Circadian Biology. “We think some factors are under circadian control. Better timing of meals, which would require a change in behavior, could be a critical element in slowing the ever-increasing incidence of obesity.”

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The findings could have implications for developing strategies to combat obesity in humans, as the United States and the world battle what has been called an “obesity epidemic.” More than 300 million adults worldwide are obese, including more than a third of American adults.

Late to bed and late to rise…

A 2011 Northwestern Medicine study confirmed these findings, and expanded upon them, finding how staying up late every night and sleeping in is a habit that could put you at risk for gaining weight.

The researchers say the problem is related to the theory that people who go to bed late and sleep late eat more calories in the evening, more fast food, fewer fruits and vegetable,s and weigh more than people who go to sleep earlier and wake up earlier.



Late sleepers consumed 248 more calories a day, twice as much fast food, and half as many fruits and vegetables as those with earlier sleep times, according to the study. They also drank more full-calorie sodas. The late sleepers consumed the extra calories during dinner and later in the evening when everyone else was asleep. They also had a higher body mass index, a measure of body weight, than normal sleepers.

“The extra daily calories can mean a significant amount of weight gain — two pounds per month — if they are not balanced by more physical activity,” said co-lead author Kelly Glazer Baron, a health psychologist and a neurology instructor at Northwestern University Feinberg School of Medicine.

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“We don’t know if late sleepers consume the extra calories because they prefer more high-calorie foods or because there are less healthful options at night,” said co-lead author Kathryn Reid, research assistant professor in neurology at the Feinberg School.

It’s not just what you eat, but when you eat

The study shows not only are the number of calories you eat important, but also when you eat them — and that’s linked to when you sleep and when you wake up, noted senior author Phyllis Zee, MD, professor of neurology and director of the Sleep and Circadian Rhythms Research Program at Feinberg.

“Human circadian rhythms in sleep and metabolism are synchronized to the daily rotation of the earth, so that when the sun goes down you are supposed to be sleeping, not eating,” Zee said. “When sleep and eating are not aligned with the body’s internal clock, it can lead to changes in appetite and metabolism, which could lead to weight gain.”

How this could help you lose weight

The research findings could be relevant to people who are not very successful in losing weight, Zee said. “The study suggests regulating the timing of eating and sleep could improve the effectiveness of weight management programs,” she said.

The findings also have relevance for night-shift workers, who eat at the wrong time of day related to their bodies’ circadian rhythms. “It’s midnight, but they’re eating lunch,” Zee said. “Their risk for obesity as well as cardiovascular, cerebrovascular and gastrointestinal disorders is higher.”

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The study included 51 people (23 late sleepers and 28 normal sleepers) who were an average age of 30. Late sleepers went to sleep at an average time of 3:45 am, awoke by 10:45 am, ate breakfast at noon, lunch at 2:30 pm, dinner at 8:15 pm and a final meal at 10 pm. Normal sleepers on average were up by 8 am, ate breakfast by 9 am, lunch at 1 pm, dinner at 7 pm, a last snack at 8:30 pm and were asleep by 12:30 am.

Participants in the study recorded their eating and sleep in logs and wore a wrist actigraph, which monitored sleep and activity cycles, for at least seven days.

Late sleepers function in society by finding jobs where they can make their own hours, Baron noted, such as academics or consultants. “They find niches where they can live this lifestyle, or they just get by with less sleep,” she said.



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