In a commentary published in this week's Canadian Medical Association Journal, two obesity researchers argue that the old formula — energy in must be lower than energy out — is too simplistic.
"We tend to always talk about food and physical activity and we need to go beyond that to include what I call other non-caloric factors," said Jean-Philippe Chaput, a specialist in preventive medicine who works on obesity in children at the Children's Hospital of Eastern Ontario, in Ottawa.
"We know that obesity is very complex. It isn't one-size-fits-all. People gain weight for different reasons. It's not always an increase in food intake. It can be stress. It can be depression. Genes. Different factors."
Inadequate sleep is among them, wrote Chaput and his co-author, Dr. Angelo Tremblay, of the department of kinesiology at Laval University in Quebec City.
It's generally recommended that adults get seven to nine hours of sleep a night.
Scientific studies are increasingly pointing to the role sleep — or the lack of it — plays in maintaining a healthy body weight. The evidence suggests inadequate sleep influences body weight in a number of ways.
People who don't get enough sleep are awake longer — no surprise there — and that gives them more time to consume calories.
"It's well known that television viewing stimulates food intake in the absence of hunger," Chaput said Monday in an interview.
Studies have found that people who stay up watching television, for instance, are likely to snack. And generally speaking, they don't nibble on steamed broccoli or celery sticks.
In fact, late night TV watching is associated with consumption of high calorie foods. Chaput says a study showed people who go to bed late eat about 400 to 500 calories more a day than people who go to bed early and wake up early.
But the influence of short sleep isn't just related to the fact that it gives people more time to eat. Studies show that people who sleep for shorter periods produce more ghrelin, a hormone that stimulates the appetite.
Inadequate sleep also puts the body under stress, creating higher levels of cortisol, a stress hormone. It's well known that low-level stress makes people eat more, Chaput said.
These hormonal changes can sabotage weight loss efforts, he suggested.
"We know that short sleepers in general feel more hungry. And when we restrict calories in the diet of short sleepers, we know that if we already feel more hungry and you cut calories, hunger plus hunger means very hungry," he said.
"If they want to lose weight, of course at some point they will need to cut some calories. But if they don't take into account their sleeping patterns, they might fail."
And there's a third factor at play, suggested Dr. Arya Sharma, who holds a chair in obesity research at the University of Alberta in Edmonton. Sharma is not an author of the commentary, but he agrees with the points it makes.
"We've ... known for a long time that people who don't get enough sleep tend to be less physically active during the day," Sharma said.
Sleep is part of the discussion when he meets with patients struggling to lose weight, Sharma said. And he welcomed the raising of the issue in the journal.
"I think the discussion is important because we tend to focus so much on what people do — how much they eat and how much they exercise — rather than looking at some of the factors that drive those behaviours," he said.
"And not getting enough sleep is certainly one of the main, key drivers of unhealthy eating behaviours, and probably also not having the energy to be more physically active."
Lack of sleep also affects mood, which has been shown to be a trigger of emotional eating, Sharma said. And maintaining impulse control is harder when a person is tired, he said.
"All of these things tie together."
Unfortunately, just telling people to go to bed earlier isn't likely to solve the obesity problem, Chaput said. There are a variety of reasons for why people don't get enough sleep, and some may resist a quick fix.
Still, he and Tremblay suggested future research should try to identify ways to help people get more sleep, such as pushing back the start time for work or for school — especially for teenagers — or scheduling prime-time TV programs earlier in the evening.