Even people who think they eat three meals a day may actually graze for most of their waking hours and consume fewer calories when they get more sleep, a small U.S. study suggests.

Researchers asked volunteers to use a mobile app to snap pictures of everything they ate and drank over three weeks. Most participants consumed food and drinks over about 15 hours of the day, taking in less than 25 percent of their calories before noon and more than 35 percent after 6 p.m.

"Most people think they eat three meals and a snack or two within a 10-12 hour window, but we found the majority spread their caloric intake over a very long time," said study co-author Satchidananda Panda of the Salk Institute for Biological Studies in La Jolla, California.

The trouble with eating or drinking over a longer stretch of waking hours and consuming more calories at night is that "it confuses our body's biological clock and predisposes us to obesity, diabetes, fatty liver disease, high cholesterol and cardiovascular disease," Panda added by email.

Because many existing apps and food diaries can encourage people to eat less just by seeing what they record, Panda and co-author Shubhroz Gill at the Salk Institute devised an app that would erase data as soon as images were logged. This meant the app should have minimal impact on how people ate, Panda said.

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Based on an analysis of snapshots recorded by more than 150 volunteers, the researchers got a sense of what people ate when, and under what circumstances.

They could see, for example, what people photographed next to a keyboard, in bed, watching television, or walking down the street.

Pictures also told a story about what people tended to favor at particular times of day. Coffee was more common in the morning, while alcohol was more likely to appear at night. People drank tea throughout the day, and images of chocolate and candy made regular appearances from about 10 a.m. onward, the study found.

The researchers also tested whether the app might help people eat less by encouraging them to consume food and drink over a shorter stretch of the day.

They asked eight overweight people who tended to eat over more than 14 hours of the day to cut back to 10 to 11 hours. After 16 weeks, these people lost about 3.5 percent of their excess body weight and reported sleeping better.

One drawback of the study is that it's too small to draw any broad conclusions about whether eating over fewer hours during the day might lead to weight loss, or whether sleeping more causes less food consumption, the authors acknowledge in Cell Metabolism.

While the study also isn't designed to prove whether mobile apps or other forms of food tracking can help with weight loss, the findings build on a large body of research linking self-monitoring of dietary habits to weight loss, said Elina Helander, a researcher at Tampere University of Technology in Finland.

"Smartphones can make self-monitoring easy compared to paper diaries," Helander, who wasn't involved in the study, said by email.

Mobile apps may also provide more real-time reminders in reaction to pictures or data supplied by dieters, though these tools still rely on people to motivate themselves to interact with the technology, said Frank Scheer, a sleep researcher at Harvard University and Brigham and Women's Hospital in Boston.

Previous research has found that consuming those first calories earlier in the day may predict greater success with weight loss, Scheer, who wasn't involved in the study, said by email.

"The current study further suggests that extending the overnight fasting duration in overweight individuals with a habitually short overnight fasting duration leads to weight loss, at least in part due to reduced daily caloric intake," Scheer said.