This 5 year study of more than 3,300 American teenagers led by Lauren Asarnow of the University of California indicated that every extra hour that teenagers stayed up during the school week added 2.1 points to the Body Mass Index (BMI). The figure was unaffected by exercise, screen time or the actual number of hours they slept. The study used data from 1994 – 2009.

http://news.berkeley.edu/2015/10/01/late-bedtimes-bmi