Eating Late at Night: The New Science of Why It’s Making You Fat

Claire Benoist for Reader's Digest

Jackie Rodriguez gained 70 pounds after her first child was born. “I was very unhappy, but I stayed like that for two years,” she says. Then, when her daughter was two, she dropped all the weight with practically no effort. “I wasn’t using any diet pills, fat burners, or shakes … nothing,” she recalls.

The transformation had nothing to do with what Rodriguez ate. Rather, it began when she started a new job that shook up her daily routine. Working in the office of a DJ company near her apartment in the Bronx in New York City, she started her shift at 5:30 p.m. Instead of sharing dinner with her husband at nine, when he got home from his job as a superintendent, she ate alone at five, before she dropped her child with a sitter and went to work.

Within nine months, she’d slimmed from a size 16 to a size 2. She felt like a movie star who seems to lose baby weight effortlessly. “You don’t think that could happen to you,” she says.

Night work often leads to weight gain, so Rodriguez’s story might seem to be a quirk of her particular physique. But unlike many such workers, who labor in the wee hours or work rotating shifts, Rodriguez clocked out by 11 p.m. and got a regular night’s sleep. Perhaps even more important, she didn’t eat at work or when she got home—just showered and went to bed.

Her main adjustment was moving dinnertime almost four hours earlier. That single, simple change seems to have triggered Rodriguez’s dramatic weight loss—and emerging scientific evidence may explain why.

New Danger of Night Eating
In labs around the world, researchers are developing a completely new understanding of how metabolism works. It seems that our bodies are primed to process food most efficiently when it’s eaten during daylight hours. “We now recognize that our biology responds differently to calories consumed at different times of day,” says Harvard neuroscientist Frank Scheer, PhD. That means a habit as innocuous as eating at night, compared with eating calorically equivalent meals during the day, may cause people to gain weight. “That late-night bowl of ice cream may all go toward your waistline,” says UCLA neuroscientist Christopher Colwell, PhD, author of Circadian Medicine.

[pullquote] Our biology responds differently to calories consumed at different times of day. [/pullquote]

Just look at Satchidananda Panda’s mice. A molecular biologist at the Salk Institute for Biological Studies in La Jolla, California, Panda is a leading expert on how the timing of food intake affects health. His team has found that mice that eat only during their active hours (the equivalent of daytime for humans) are drastically healthier and thinner than mice that eat the same amount of food scattered over 24 hours.

Encouragingly, when unhealthy, snack-around-the-clock mice are put on a strict schedule that allows them to eat only during their daytime, their diabetes and fatty liver disease improve and their cholesterol levels and inflammation markers diminish. “It’s likely we can reduce the severity [of disease] just by changing when people eat,” Panda says.

The Food-Driven Clock
To understand the connection between meal timing and health, you have to go way, way back in history. The dramatic daily shifts between light and darkness on our planet because of sunrise and sunset have been incorporated into the biology of nearly every living thing. Our internal organs function differently during the day from how they do at night, in patterns known as circadian rhythms.

Over the past few years, researchers have discovered that unnatural light exposure—such as staying up late amid the glare of a digital screen—disrupts these rhythms in ways that over time can lead to a host of illnesses.

But now experts have begun to suspect a second circadian clock in the body—organized around food, not light. Scientists still have much to learn about this food-based body clock, but evidence suggests that round-the-clock snacking may pose as much of a danger to our health as artificial light at night. Night eating has been implicated as a factor in diabetes, heart disease, cancer, and learning and memory problems.

Throughout evolution, daytime has been for nourishment and nighttime for fasting, and our organs have evolved accordingly. Digestive enzymes and hormones ebb and flow in a predictable pattern over the course of 24 hours, enabling the liver, intestines, and other digestive organs to function together as one well-oiled machine. Our modern world of late-night takeout and snack-filled pantries threatens to upend this calibrating role of food.

“When you eat all the time, your insulin and glucose levels are elevated all the time,” says Ruth Patterson, PhD, a nutrition expert and epidemiologist at the University of California, San Diego.

[pullquote] Night eating has been implicated as a factor in diabetes, heart disease, cancer, and learning and memory problems. [/pullquote]

Insulin promotes growth—its constant presence in the bloodstream may give precancerous cells a deadly boost. In new research on breast cancer survivors, Patterson and her colleagues found that breast cancer recurrences were less likely when women abstained from food for at least 13 hours at night.

Gut Rest: How It Works
Compared with other kinds of diets, night fasting is simple. In a small pilot study, Patterson’s team told women to eat dinner as early as 6 p.m. and definitely by 8 p.m. and not to eat again until eight in the morning, for at least 12 hours of “gut rest.”

“[Fasting] they instantly understood,” Patterson says. “They didn’t have to change what they ate or how they cooked. They would say, ‘If I give my husband a salad for dinner, that doesn’t always fly.’ But when they just said, ‘I don’t ever eat after eight o’clock,’ the men were like, ‘Whoa, tough girl!’ They got respect.”

The new research suggests that breakfast really is the most important meal of the day—but we need to embrace its original meaning: breaking a fast. The first meal of the day is most beneficial only if it comes after 12 to 14 hours of not eating or drinking, says Panda.

In addition to fasting at night, it’s beneficial to eat your main meal earlier in the day. In a 2013 study, Harvard’s Frank Scheer and Marta Garaulet of the University of Murcia in Spain analyzed 420 dieters at weight-loss clinics. Participants ate the same number of calories and were equally active, but those who had their main meal before 3 p.m. lost significantly more weight than those who ate later. “To find such big differences in weight loss with just a slight difference in meal timing is quite remarkable,” says Scheer.

To many, the science of meal timing is nothing but common sense. Craig Weingard, a compliance manager at a financial firm, is an acolyte of a bodybuilding expert who for years has included nightly fasts among his recommendations. For the longest time, Weingard resisted. It seemed too painful to go to bed hungry. Finally, he tried it. “In a flash, my whole body changed. I literally can see it the next day when I look at my stomach if I didn’t eat after six,” he says. “Anything you eat after 6:15 p.m. becomes part of you.”

Tricks to Use the Food Clock to Lose Weight

1. Fast for at least half of each day. Try not to eat for at least a 12-hour span daily. Ideally your fast would begin after the evening meal (from 6 to 8 p.m.) and extend until breakfast (8 a.m.).

2. Eat breakfast like a king and dinner like a pauper. A 2013 Israeli study put overweight and obese women into two groups. Both had the same number of calories, but one ate a large breakfast, a medium lunch, and a small dinner; the other had a small breakfast, a medium lunch, and a large dinner. The large-breakfast group lost more weight and showed a stronger improvement in metabolic health metrics.

3. Forgo late-night noshing. A 2015 study found that an evening meal raises people’s blood sugar levels 17 percent more than does an identical meal eaten in the morning. Related research found that the number of calories people burn digesting food in the first two hours after a meal drops by half if they eat the meal in the evening versus the morning

4. Consume only water during your fast. Anything else will start your body clock. Put off that morning coffee until after your 12-hour window.

5. Adjust to your natural rhythms. Early birds might want to eat supper at 6 p.m. and fast until 6 a.m. or later. For night owls, it might be easier to have dinner at 9 p.m. and fast until at least 9 a.m.