Each extra hour per day spent watching TV, using the internet or playing computer games during Year 10 is associated with poorer grades at secondary education examination at age 16, finds a study from University of Cambridge.
The researchers also found that pupils doing an extra hour of daily homework and reading performed significantly better than their peers.
However, the study found that even if the participants did a lot of reading and homework, watching TV or online activity still damages their academic performance.
Also, the level of physical activity had no effect on academic performance.
The team studied 845 pupils from secondary schools in Cambridgeshire and Suffolk, measuring levels of activity and sedentary behaviour at age 14.5 years and then comparing this to their performance in their examinations the following year.
The team found that screen time was associated with total General Certificate of Secondary Education (GCSE) points achieved.
Each additional hour per day of time spent in front of the TV or online at age 14.5 years was associated with 9.3 fewer GCSE points at age 16 years - the equivalent to two grades in one subject (for example from a B to a D).
Two extra hours was associated with 18 fewer points at GCSE.
"Spending more time in front of a screen appears to be linked to a poorer performance at GCSE," said first author Kirsten Corder.
The findings were published in the open access International Journal of Behavioural Nutrition and Physical Activity.
