Teenagers need about nine hours of sleep a night—but they get closer to seven. And around puberty, their circadian clocks shift by a couple of hours, meaning they get tired later at night than before and wake up later in the morning than they used to. This shift reverses at adulthood. The biological nature of this daily rhythm means that sending a teenager to bed earlier won’t necessarily mean they fall asleep earlier.

I agree with the phone part though.