Thanks - though that still doesn't offer any details on why they believe there is a biological cause, not just a social cause. > Adolescents are increasingly u

Thanks - though that still doesn't offer any details on why they believe there is a biological cause, not just a social cause. > Adolescents are increasingly using stimulants to compensate for sleep loss, and caffeinated and/or sugary drinks are the usual choice. The half-life of caffeine is 5 to 9 hours. So a caffeinated drink late in the day delays sleep at night. Tiredness also increases the likelihood of taking up smoking. > Collectively, a day of caffeine and nicotine consumption, the biological tendency for delayed sleep and the increased alertness promoted by computer or cellphone use generates what Carskadon calls a "perfect storm" for delayed sleep in teenagers. Having looked at the full paper about school start times from AAP [1] it cites three papers on this subject [2] [3] [4], I'll come back with another comment if I manage to dive deeper into the rabbit hole on those... (I'm not saying there isn't a biological cause, just that since my gut instinct is that it would be caused by cultural reasons I'm interested to see the reasoning behind thinking there are biological reasons too.) [1] http://pediatrics.aappublications.org/content/early/2014/08/19/peds.2014-1697 http://pediatrics.aappublications.org/content/early/2014/08/... [2] Frey S, Balu S, Greusing S, Rothen N, Cajochen C. Consequences of the timing of menarche on female adolescent sleep phase preference. PLoS ONE. 2009;4(4):e5217 [3] Carskadon MA, Acebo C, Seifer R. Extended nights, sleep loss, and recovery sleep in adolescents. Arch Ital Biol. 2001;139(3):301–312 [4] Roenneberg T, Kuehnle T, Pramstaller PP, et al. A marker for the end of adolescence. Curr Biol. 2004;14(24):R1038–R1039