Loath as you may be to admit it, chances are that at some point you have found yourself in the kitchen late at night, devouring some sweet, salty or carb-rich treat even though you weren’t hungry.
Scientists are getting closer to understanding why people indulge after dark and to determining whether those nighttime calories wreak more havoc – whether they drive up the risk of weight gain and of chronic diseases such diabetes – than ones consumed earlier in the day.
“For years, we said a calorie is a calorie no matter when you consume it,” says dietitian Joy Dubost, a spokeswoman for the Academy of Nutrition and Dietetics. “I don’t know if we can say that anymore, based on the emerging research. The timing of a meal may potentially have an impact.”
Most of the major studies on late-night eating have been conducted with animals, night-shift workers and people who, due to a disorder called night eating syndrome, consume at least 25 percent of their daily calories after supper or who wake up to eat at least twice a week.
Studies tend to show that when food is consumed late at night – anywhere from after dinner to outside a person’s typical sleep/wake cycle – the body is more likely to store those calories as fat and gain weight rather than burn it as energy, says Kelly Allison of the University of Pennsylvania School of Medicine’s Center for Weight and Eating Disorders.
Some animal studies have shown that food is processed differently at different times of day. This could be due to fluctuations in body temperature, biochemical reactions, hormone levels, physical activity and absorption and digestion of food, says Steven Shea, director of the Oregon Institute of Occupational Health Sciences at Oregon Health & Science University.
“The studies suggest that eating out of our normal rhythm, like late at night, may prompt weight gain” and higher levels of blood sugar, which can raise the risk of chronic disease, Allison says.
Not enough research on what prompts weight gain has been done, Allison says, to determine whether timing is as important as – or even more important than – the types or amounts of food often consumed at night. People tend to choose more highly palatable items – sweet and salty foods, which tend to be more caloric – when they’re tired and have restrained themselves all day, Allison adds. And night-shift workers tend to overestimate how many calories they need to stay awake while on duty.
Two recent studies have shed new light on the potential impact of timing. In a study of 420 overweight or obese people published in 2013, those who ate their major meal after 3 p.m. lost less weight during a 20-week weight-loss program than those who ate that main meal before 3 p.m. – even when the amount they ate, slept and exercised was the same.
“This is the first study to show that eating later in the day … makes people lose less weight, and lose it slower,” even when the amount people ate, slept and exercised was the same, says the study’s lead author, Marta Garaulet, a professor of physiology at the University of Murcia in Spain. “It shows that eating late impairs the success of weight-loss therapy.” In the 2013 study, the early eaters lost 22 pounds, the late eaters only 17.
In a subsequent small study of healthy women published this year, Garaulet and her team showed that when participants ate lunch after 4:30 p.m., they burned fewer calories while resting and digesting their food than they did when they ate at 1 p.m. – even though the calories consumed and level of activity was the same.
What’s more, when the participants ate late, they couldn’t metabolize, or burn off, carbohydrates as well as when they ate earlier. They also had decreased glucose tolerance, which can lead to diabetes. (The two-week study did not track whether the women gained or lost weight.)
“There are still many questions to answer,” Garaulet says, and further study should focus on “clock” genes in fat tissue that can affect metabolism.
She is exploring what happens if you eat at the wrong time for your body-fat clock – i.e., at a time when your fat tissue is not ready for it. “It could be that if you eat late, then the capability your body has to mobilize [and burn] fat is lower because it’s not the right time,” Garaulet says.
Evolution at work?
Most Americans spurn the adage to “eat breakfast like a king, lunch like a prince and dinner like a pauper.” U.S. adults consume 17 percent of their day’s calories at breakfast, 24 percent at lunch and 34 percent at dinner, according to the USDA’s “What we eat in America” survey.
It may be the pace of work life, which leaves room for little more than a quick breakfast and lunch during the typical weekday.
But circadian rhythms – the internal body clock that regulates sleep and other cycles based on light and darkness – may also be a factor. In a small study published in 2013, a group of non-obese adults stayed in a dimly lit area for 13 days, got plenty of sleep and consumed identical meals at even intervals throughout 24-hour periods. Despite that regularity, they still reported being substantially hungrier at 8 p.m., than they were at 8 a.m. They also had more cravings for sweet, salty and starchy foods in the evening.
The Oregon Institute’s Shea, who co-authored this study, suggests that evolution may be at work. For our primal ancestors when food was scarce, he hypothesizes, “one of the most [evolutionarily] efficient things to do in the evening was to eat. That’s when the body can store energy as fat and glycogen, so that you’re ready for what might happen the next day without having to immediately replenish calories by eating.”
Hormones may also be driving us to eat late, says Pamela Peeke, an assistant professor of medicine at the University of Maryland School of Medicine.
Cortisol and adrenaline, two hormones that follow the natural circadian rhythm, plummet by the time 3 p.m. rolls around, as do energy levels, as the body prepares for the end of the day. That is fine if you’re shutting down, too, and planning on a 5 p.m. dinner and then early bedtime. That energy drop off is not so fine if you’re still working or rushing to meet a late-day deadline.”Focus begins to wax and wane, and that’s when people start making mistakes,” Peeke says. Instead of heeding the body’s signals to get an early dinner and then get to bed, many people head to the vending machine or the coffee shop for an energy boost.
And the high-sugar, high-fat foods they reach for only rev up the appetite, possibly setting the scene for a late-night food bender. “What you’re doing is compounding a mess,” Peeke says. “If you eat junk, that’s going to jack up your insulin and drive you to forage for more sugar later on.”
But not all late-night eating may be bad. Some researchers are exploring whether there is an upside to a small late-night snack.
A number of recent small studies have shown that consuming a 150-calorie protein shake 30 minutes before bed may help muscles grow, quell morning appetite, boost metabolism, help the body recover from tough workouts and have other positive effects. In one study, 44 healthy young men who had a protein shake before bed gained more strength and muscle mass from a three-month resistance training routine than those who did not.
“Sleep is the only time you’ve got when you’re not doing other things requiring energy,” says Michael Ormsbee, director of the Institute of Sports Sciences & Medicine at Florida State University. “When you totally shut down almost every other action other than staying alive, the body is primed to work on recovery, cell turnover, improving immune function and repairing and regenerating sore and damaged muscle tissue.”