What's the best time of the day to lose weight?

By Dr. Ananya Mandal, MDNov 8 2018

Image Credit: WindNight / Shutterstock

Researchers have found that late afternoons and early evenings are the best time to lose weight as the metabolic rates are the highest around that time. The results of the study titled 'Human Resting Energy Expenditure Varies with Circadian Phase', is published in the latest issue of the journal Current Biology.

For this study the seven participants (aged 38 to 69 years) were made to stay inside a special laboratory with no clues about time of the day for 37 days. They were not given access to windows, phones, clocks or the internet. Each night the bedtime for the participants was shifted by four hours. It was similar to be travelling across four time zones to the west each day for three continuous weeks. Their sleep wake cycle as well as food and exercise was also tightly regulated by the researchers. This helped the researchers alter the body’s circadian rhythm or clock. Now metabolism and metabolic rates at different times of the day was analyzed. The study results revealed that 10 percent extra calories could be burnt in the late afternoons and early evenings as the metabolic rates are the highest around that time. The body temperatures were lowest around middle of the night and highest in the late afternoons and early evenings.

According to the team of researchers, this study shows why shift workers tend to be irregular in their sleep and eating schedules and tend to gain weight. Shift workers are at a greater risk of diabetes, heart disease, obesity, cognitive problems as well as cancers, as has been seen in several previous studies. The researchers explain that the body’s circadian clock could be governing the metabolism and metabolic rates.

Kirsi-Marja Zitting, of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School in Boston, one of the team members said that they were surprised that same exercises and exertions at one time of the day worked at burning calories differently than when performed at a different time of the day. “Because they were doing the equivalent of circling the globe every week, their body’s internal clock could not keep up, and so it oscillated at its own pace,” Zitting said.

Jeanne Duffy, co-author of the study and an associate professor of medicine at Harvard Medical School and a neuroscientist at Brigham and Women’s Hospital, added that this study showed that it was not only what we ate and how much we exercised that determined how many calories we would burn but the “when” was also very important. “Regularity of habits such as eating and sleeping is very important to overall health,” she said. She explained that 10 percent extra calories could be burnt if exercised in the later afternoons and early evenings compared to middle of the night. It means that a person can burn 130 extra calories with no added effort, she said. She went on to explain that if this was happening each day, the resultant accumulative effects can also be estimated. It is thus important to keep the body’s internal clock in synchrony with the external environment for good heath she said. “Regularity is really important,” she said.

As the next step the team would connect the effects of sleep time, duration and regularity and its effect on weight gain.