In 2015 Cambridge University researchers recorded the activities of more than 800 14-year-olds and analysed their GCSE results at 16. Those spending an extra hour a day on screens (TV, computer, games console, phone) saw a fall in GCSE results equivalent to two grades overall.
On average, the 14-year-olds said they spent four hours of their leisure time each day watching TV or in front of a computer.
An additional hour of screen-time each day was associated with 9.3 fewer GCSE points at 16 - the equivalent of dropping a grade in two subjects.
Two extra hours of screen-time was associated with 18 fewer points - or dropping a grade in four subjects. Even if pupils spent more time studying, more time spent watching TV or online, still harmed their results, the analysis suggested.