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(CNN) -- If your main goal is to lose weight, you may want to consider the time of day you exercise. A new study offers clues that could help maximize your efforts.

According to a study published Tuesday in the academic journal Obesity, exercising between 7 a.m. and 00 a.m. is associated with having a lower waist circumference and body mass index than people who exercise at noon or at night.

"This is exciting new research that is consistent with common advice for reaching exercise goals: Scheduling exercise in the morning, before distracting emails, phone calls or meetings," Rebecca Krukowski, a clinical psychologist with an expert in behavioral weight management who was not involved in the study, said in a news release.

Other researchers had already described positive links between moderate and vigorous physical activity and weight loss. However, findings about the best time to exercise and lose weight have been mixed, so the authors of the latest study looked at what influence doing activity at different times might have on the relationship between exercise and obesity.

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The authors studied health and activity data from 5,285 people who had participated in the 2003 to 2006 cycles of the U.S. Centers for Disease Control and Prevention's National Health and Nutrition Examination Survey. The researchers chose those specific years because that's when accelerometers, or activity trackers, were first used in the survey.

After recording the participants' BMI and waist circumference, they placed an activity tracker on the right side of their hips during waking hours, 10 hours or more a day, for four to seven days.


People who exercised in the morning, between seven and nine, had an average BMI of 27.5, compared to those who exercised at noon and in the evening, who had an average BMI of 28.3. The average waist circumference, adjusted for diet quality and calorie intake, was 96 centimeters, 97.8 centimeters, and 97.5 centimeters, respectively.

These results held regardless of gender, ethnicity, education, tobacco use, alcohol consumption or sedentary behaviour. What's more, even among people who met physical activity guidelines (at least 150 minutes a week), reaching this goal in the morning was associated with lower BMI and smaller waist size.

"This research provides valuable data based on a national sample of U.S. participants, something that has not been done before on exercise timing and weight loss," Dr. Tongyu Ma, lead author of the study and assistant professor of rehabilitation sciences at Hong Kong Polytechnic University, said by email.

However, participants' BMI and waist size were measured before the activity follow-up period, and not remeasured afterward, so the authors could not prove that exercising in the morning directly influenced any of the measurements.

Ma plans to conduct further studies to confirm the results and determine whether there is a cause-and-effect relationship between exercising in the morning and achieving a lower BMI and waist contour.

Why time can matter

According to experts, the reasons for these results may have to do with both physiology and lifestyle habits.

Because of the study design, "it's not known whether people who exercise consistently in the morning can be consistently different from those who exercise at other times, in ways that weren't measured in this study," said Krukowski, a professor of public health sciences and co-director of the Center for Community-Based Health Equity at the University of Virginia School of Medicine.

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"People who exercise regularly in the mornings may have more predictable schedules, such as being less likely to work shifts or less likely to have caregiving responsibilities that prevent morning exercise," he added. "Predictable schedules could have other advantageous effects on weight that were not measured in this study, such as with sleep duration (or) quality and stress levels."

In addition, early risers may be biologically different from night owls, experts said.
The authors noted that, based on previous studies, people who exercise in the morning are more likely to have a lower daily caloric intake and to spend more energy passively when not exercising. It may sound counterintuitive, but this trend is likely because exercising in the morning on an empty stomach means that the body relies on stored fat for energy, rather than glucose stored in food. This may mean that early risers' bodies are better equipped to increase oxidation, or fat burning, both during exercise and the next day, even if they are sedentary after training.

In the latest study, people who exercised in the morning were the most sedentary despite having the lowest BMI and waist circumference.

"Personally, I like it because it tells me that if I exercise in the morning, I can sit in the office and focus on work the rest of the day without worrying about weight gain," Ma explains via email.

According to the authors, you can also lose more weight by doing more concentrated or structured exercise, another pattern they found in the morning group.

If it can be done, "aerobic exercise first thing in the morning — like biking, running, or even brisk walking to begin with — is a promising weight-loss tool," Ma says. "In fact, most people are more likely to stick to a morning exercise routine than midday or evening exercise."

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