Study identifies worrying trends between teenage screen use and lower physical activity Two thirds of children use more than one screen at the same time after school, in the evenings and at weekends as part of increasingly sedentary lifestyles, according to new research at the University of Leicester. An NIHR study of more than 800 adolescent girls between the ages of 11 and 14 identified worrying trends between screen use and lower physical activity - including higher BMI - as well as less sleep. The use of concurrent screens (termed 'screen stacking') grew over the course of the week - with 59% of adolescents using two or more screens after school, 65% in the evenings, and 68% at weekends.