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Exposure to the sun's natural light is certainly a good idea for weight loss. Scientists say that people who walk outdoors in the morning are slimmer than those who walk outdoors later. They believe bright morning light helps synchronize the body clocks, which helps regulate the body's metabolism.

American researchers let 54 men and women wear wrist-like monitors to record their body exposure to light, sleep status and physical activity within a week. A diary was also kept of what the volunteers ate for a week. The results of the calculation and analysis showed that even after taking into account factors such as exercise, diet, and sleep, the weight of those exposed to bright light in the morning was still relatively light.

The timing, intensity and duration of natural light all matter, say researchers at Northwestern University Feinberg School of Medicine in Chicago. They say it is difficult to get enough natural light indoors and advise us to get outside, even on cloudy days. Just 20 minutes to 30 minutes is enough.

Dr Phyllisz, senior author of the study, said: "Natural light is the most effective way of synchronizing your body's circadian clock, which in turn plays a role in regulating energy balance.

"If a person doesn't have enough exposure to natural light at the most suitable time of day for light, maybe your body clock can't get in sync. Everyone knows that metabolic changes in the body can lead to weight gain. The key is that you have to be at 8 a.m. Expose yourself to bright natural light between noon and noon."

Researcher Dr Catherine Reid said: "It's like people are trying to get more sleep to help them lose weight. Maybe getting more exposure to natural light is another way to lose weight."

She suggested that office workers take advantage of the lunch break to go outdoors to get more exposure to natural light, and at the same time improve artificial lighting in schools and workplaces.

It's not entirely clear how well bright morning light can help us lose weight, but natural light is known to affect metabolism, hunger and satiety in the body, according to the report published in the journal PLOS ONE .