Some articles on the importance of recess time, time outdoors, regular exercise, and free play for kids (credit to Scarymommy.com for the links):
https://www.scarymommy.com/need-recess-wet-cold-weather/
https://www.cdc.gov/healthyyouth/health_and_academics/pdf/pa-pe_paper.pdf
https://www.scarymommy.com/school-recess-should-be-mandatory/
https://www.ncbi.nlm.nih.gov/pubmed/19171606
http://pediatrics.aappublications.org/content/131/1/183
https://www.scarymommy.com/lack-recess-affects-kids-mental-health/
