There’s a long-held belief that weight is controlled by a person’s genetics, diet and exercise habits, but scientists this week have made a surprise finding.
Reporting in Current Biology on Thursday, US researchers found the number of calories or kilojoules people burn while at rest changes with the time of day.
When at rest, study participants burned 10 per cent more calories in the late afternoon and early evening than in the early morning hours – a finding that suggests energy burn is dictated not only by what we eat, but also when we eat and sleep.
“There is now emerging evidence that an irregular sleep-wake and fasting-feeding cycle … can lead to disrupted circadian timing, which in turn may alter energy balance and lead to increased obesity risk,” the study authors wrote.
To test this theory, the researchers subjected seven people to a somewhat lab rat environment – where the participants had no clocks, windows, phones or internet for three weeks.
The participants didn’t know what time it was outside, and they were assigned to go to bed and wake up at set times. Each night, those times were adjusted four hours later, the equivalent of travelling west across four time zones every day.
Over the three weeks, the researchers monitored the participants’ energy and resting levels to determine how the body’s internal clock – our circadian rhythm – controlled their metabolism and energy output.
Diet and exercise types were not controlled during the study, which may have affected the results.
“The fact that doing the same thing at one time of day burned so many more calories than doing the same thing at a different time of day surprised us,” lead study author Kirsi-Marja Zitting from Brigham and Women’s Hospital and Harvard Medical School said.
Not that simple
Before you start planning your meals around the 3pm slump, it’s important to look at the bigger picture, one expert told The New Daily.
Edith Cowan University exercise and sport science lecturer Dr Krissy Kendall said the majority of research still supported that eating earlier in the day was more beneficial than eating later.
“There’s interesting information in this study, and it was done well,” Dr Kendall said.
“Because it was such a small study it’s very difficult to understand why they observed what they did.
“However, there are several other articles that show that when you consume a meal earlier in the day, you have an increase in your metabolic rate, and improved insulin sensitivity, which means there is less glucose in the blood.
“You also are able to process the food better in the morning, compared to the food you consume at night.”
Getting adequate sleep, about eight hours each night, is equally as important, Dr Kendall said.
Numerous studies, particularly on night-shift workers, have shown that people who eat their larger meals later in the day, and have disrupted sleep cycles, are more likely to put on more weight, and have poor insulin sensitivity.
Dr Kendall said this was why most nutritionists recommend eating your bigger meals earlier in the day.
“Let’s say you had the exact same meal for breakfast and same meal for dinner, your body has a higher resting energy expenditure post-morning feeding than it does eating late at night,” she said.
“That’s not to say you should stop eating at night. But you should be having smaller meals later on, because your body doesn’t have the same response to food in the evening as it does in the morning.”
Down to the individual
The other piece of the puzzle is exercise. But, studies show there is no right or wrong time when it comes to working out.
“It’s all about personal preference,” Dr Kendall said.
“If you tend to work out better in the morning, work out in the morning.
“If you’re somebody who’s never exercised before, we see benefits in training in the afternoon. But the body can adjust over time, so it’s about finding out what works for you.”
The study authors have said they are now looking at how appetite and the body’s response to food varies with the time of day.
But for now, conventional wisdom on weight loss prevails.