Screens can be addictive, as any parent trying to tear their child from games on a smartphone knows. The biggest study of youth exercise does not give that as the reason for an alarming trend towards sedentary behaviour, but it is surely a contributor. Some 1.6 million children aged between 11 and 17 from 146 countries and territories were surveyed by the World Health Organisation between 2001 and 2015. The alarming conclusion was that four in five fail to get the recommended one hour a day of moderate to intense physical exercise.

Parents pushing for their sons and daughters to be the best in the class or to get into top schools may not see such a result as a problem. They presumably would prefer their children to be indoors studying or doing extracurricular activities such as music lessons than being outside. But the United Nations agency, which is working for better global health, worries about the rising incidence of obesity, type 2 diabetes and high blood pressure. It makes a point that physical exercise is closely tied to better heart and respiratory functioning, bones and muscles, sleep, mental health and cognitive activity, which have implications for student learning. For at least an hour a day, adolescents should be walking, cycling or doing sports.

Hong Kong was not among the places studied, but South Korea was; it came out worst, with 94 per cent of teenagers not meeting the goal, in keeping with a trend of high-income Asia and Pacific societies having the world’s worst levels of activity. Even the best, Bangladesh, fared poorly, with just 34 per cent doing the minimum of exercise. There was also a marked gap between the sexes, boys getting considerably more physical activity than girls. Globally, there was no obvious pattern of inactivity between regions and a population’s income.