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 per cent 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 3pm lost less weight during a 20-week weight-loss program than those who ate that main meal before 3pm - 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 (10kg), the late eaters only 17 (7kg).
In a subsequent small study of healthy women published this year, Garaulet and her team showed that when participants ate lunch after 4.30pm, they burned fewer calories while resting and digesting their food than they did when they ate at 1pm - even though the calories consumed and level of activity was the same.
What's more, when the participants ate late, they couldn't metabolise, 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 mobilise [and burn] fat is lower because it's not the right time," Garaulet says.
Most Americans spurn the adage to "eat breakfast like a king, lunch like a prince and dinner like a pauper." US adults consume 17 per cent of their day's calories at breakfast, 24 per cent at lunch and 34 per cent 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 8am 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 hypothesises, "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 3pm 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 5pm 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."
In a 2014 study, Ormsbee and a team of researchers found that when overweight and sedentary women had a 150-calorie protein or carbohydrate shake before bedtime, they were less hungry in the morning but they also had higher levels of insulin and glucose in their blood - which, over time, can cause excessive fat storage and even diabetes. But in subsequent research published this year, overweight women who consumed a protein shake and also started exercising three times a week got the same benefits - they felt less hungry - but the insulin spikes disappeared.
"Exercise is absolutely the game changer," Ormsbee says. "You've got to include that in your daily routine."
The participants in this study didn't lose weight over the four weeks that they were tested, but longer studies are in the works to explore whether protein nightcaps might help people shed pounds.
The hypothesis is that by having a small protein snack before bed, you keep a constant influx of protein in your blood, so it can help build and repair muscle tissue while you're sleeping, Ormsbee says. And since the body has to burn calories to digest the food, there's a chance it might keep the metabolism a little more revved up overnight.
Ormsbee cautions that people shouldn't take his findings as license to have a meal before bedtime. After all, the research has involved only small, portion-controlled shakes. "We're not saying, 'Eat anything you want,' " he says.
To stop your diet from getting derailed at night:
• Don't restrict what you eat so severely during the day, says Traci Mann, a professor of psychology at the University of Minnesota and author of Secrets From the Eating Lab: The Science of Weight Loss, the Myth of Willpower and Why You Should Never Diet Again. That way, you won't have to control yourself as much at night, and you won't be preoccupied with feeling hungry and rebound with food you've been forbidding yourself to eat. "Most dieters say that their toughest time of day is post-dinner," Mann says.
• Keep junk food out of the house. Don't buy tempting indulgences: If they're not in your cupboard, refrigerator or freezer, you can't surrender to them when you go foraging at 10pm. "If it's not there, you can't eat it," Mann says.
• Eat early. Eat your main meal earlier in the day if you can: Lunchtime is better than dinnertime, says Steven Shea, director of the Oregon Institute of Occupational Health Sciences at Oregon Health & Science University. And stop all eating about three hours before bedtime, says Washington-based dietitian Joy Dubost.
• Keep after-dinner snacks small. Limit yourself to 100 to 200 calories, Dubost says.
• Don't go cold turkey. If you try giving up all sweets and alcohol all at once and promise to exercise an hour a day, you are probably setting yourself up for failure. "That's too much," says Heather McKee, who teaches behaviour change psychology at Britain's St. Mary's University, Twickenham. "You cannot cope with that. You have to take measured steps. And take it slow."
• Take good notes. Keeping a journal and tracking what, how much and why you eat can help you foster the awareness that will ultimately help you resist temptation, McKee says. "If you track your lapses and understand your triggers, you're more likely to overcome them," she says. "Awareness is the first step."
Van Allen is a writer based in Portland, Maine, who has finished 49 marathons and ultramarathons. She is the author of Run to Lose: A Runner's Guide to Weight Loss, due out in December.