It was an 80-year-old volunteer who prompted Elaine Wylie, the then headteacher of St Ninians primary school in Stirling, to reappraise the fitness of the children in her school. “Your children are not fit,” the volunteer told her. Wylie was stung by the comment, even though she suspected it was true. The PE teacher confirmed it. “Most children these days are exhausted by the warm-up,” she said.

That same afternoon, Wylie decided to send a PE class she was supervising outside to see if they could run around the playing field. “By the end, most of them were doubled up and had a stitch,” she says. “It was a shocking sight.”

Afterwards, Wylie asked the class how they thought they had done. They admitted that they were terrible – a pivotal moment because, from that point on, the children took ownership of the problem. Together, they decided that they would go outside for 15 minutes every day and see if they could build up their fitness. That was in February 2012. A month later, almost all of them could run for 15 minutes without stopping. By the summer, every class was doing it and, soon afterwards, the nursery children joined in, too. The Daily Mile had been born.

Since then, this concept of sending children outside during normal lesson time to run or walk laps of the playground for 15 minutes a day has spread to more than 3,600 primary schools in 35 different countries. Yet, while there has been plenty of anecdotal evidence about its benefits, until now hard data has been lacking. Earlier this month, a study was published that looked at 391 children at two Scottish primary schools. Seven months after starting the Daily Mile, children who had been doing it could run 5% further during a timed shuttle-run test than the other children. They had increased their moderate/vigorous physical activity by nine minutes a day and had cut their total sedentary time by 18 minutes.

Facebook Twitter Pinterest Elaine Wylie … where it all started. Photograph: Stuart Wallace

The children also experienced a 4% reduction in the size of their skin folds, suggesting that they were becoming leaner. This is important because 30% of children between the ages of seven and 11 in England and Scotland are overweight or obese; this is associated with a greater risk of diabetes and cardiovascular disease in later life. “Children often learn about healthy eating and the benefits of physical activity at school, but the kids who are doing the Daily Mile aren’t just learning it in their minds; they are learning: ‘This is something I do every day, as part of my day, and this is how it makes me feel,’” says Naomi Brooks, a senior lecturer in sport at the University of Stirling, who led the study.

Even so, initiatives such as the Daily Mile are only part of the solution. The Department of Health says that children over the age of five should be engaging in at least an hour of moderate to vigorous exercise every day – yet only a fifth of children aged between five and 15 achieve this. Moderate activities could include walking to school, riding a scooter, or cycling, while vigorous ones include playing chase, football, dancing or swimming.

If that sounds like a lot to you, you are not alone: in a recent nationwide survey, a third of parents underestimated how much exercise children need. “Parents and children felt that the biggest barrier to being more active was a lack of time,” says Susan Coan at Leeds Beckett University, who led the study. “From our research in this area, the main piece of advice would be for families to find ways of being active together that work for them. Small changes are more sustainable and can make a real difference – for example, walking part of the way to school or playing active video games as part of children’s screen time.”

One reason exercise is considered so important for children is because it helps establish lifelong habits. “If you are generally active as a youngster, it has a moderately positive effect in terms of your intention and commitment to being active as an adult,” says Prof Craig Williams, director of the children’s health and exercise research centre at the University of Exeter. There are also other long-term consequences. Late childhood and early adolescence are critical times for laying down bone, which reaches its peak density in our 20s, declining thereafter. The higher your bone density during youth, the lower your risk of fractures and osteoporosis in later life. “The idea is that we try to put as much bone as possible ‘in the bank’ as youngsters,” says Williams. “The mechanical stimulation of our body weight going through our bones, muscles and tendons when we jump up and down, stimulates our bone cells to grow and lay down new bone.”

There is also some evidence that the fitter you are when you are 18, the less likely you are to have cardiovascular problems, including heart attacks, in later life.

Even so, some question the logic of reducing children’s lesson time by 15 minutes each day; surely they could find ways of being more active at other times? Yet, according to a consensus statement released by Williams and 23 other child-health experts in 2016: “Time taken away from lessons for physical activity is time well spent and does not come at the cost of getting good grades. Physical activity has been found to boost young people’s brain development and function, as well as their intellect.” Indeed, in a previous study, Brooks found that a single bout of exercise left children feeling more awake, increased their attention and verbal memory and improved their feelings of wellbeing.

The fact that the Daily Mile takes place outside could enhance these effects further. “Bright light directly influences and activates areas of the brain that control alertness and cognition,” says Dr Rachel Sharman, a sleep researcher at the University of Oxford. It can also tweak the timing of the body clock – exposure to bright light during the morning tends to advance the clock, meaning people wake and feel sleepy earlier. “In primary schoolchildren, exercising outdoors for 15 minutes is unlikely to have much effect because they are already relatively early birds. However, at puberty, the circadian clock delays,” Sharman explains. “In teenagers, an early morning mile could pull the clock a little earlier, waking the teen up, when their bodies are – from a circadian rhythm perspective – still asleep. In terms of learning, we would then expect the teen to be in a better place, cognitively, to learn in those first few lessons of the day. Equally, by pulling the clock earlier, a teenager may then find that their body clock signals when it’s time for bed earlier the next night, meaning they get more sleep.”

In the six years since its launch, the Daily Mile has been adopted by half of Scottish primary schools and a quarter of English ones. Such is its success, that the Daily Mile Foundation has just launched an adult version, with the hope of inspiring even the busiest of people to incorporate 15 minutes of self-paced walking, jogging or running into their daily lives. As for the children of St Ninians, they continue to run outside for 15 minutes a day in all weathers, even though Wylie left the school two years ago. Last year, the primary 6 girls (the equivalent of year 5 in England and Wales) won the Scottish schools cross-country championship.

Wylie’s intention was simply to get the children fit, but what has pleased her most is the levelling effect it appears to have: “It seems that the less fit and more overweight the kids are at the start, the more they benefit,” she says. Given that obesity is more prevalent among lower socioeconomic classes, interventions such as the Daily Mile could help to close the gap in health inequality between rich and poor. If true, 15 minutes a day would be a small price to pay.