The fact that teenagers get less sleep than they should is old news; study after study over the last 30 years confirm it. In the latest research from Columbia University, published this month in the journal Pediatrics, two things stand out: it keeps getting worse, and - even more than smart phones and iPads - America's added fat is to blame.

At least a third of teens (varying by age, 15 the worst spike) don't get the nine hours of sleep doctors say they need each night. Inadequate sleep patterns profoundly and negatively affect their judgment and decision making, grades, drug and alcohol abuse, physical health, moods and mental health, and driving/safety. Of the 270,000 eighth- to 12th-graders asked nationwide between 1991 and 2012, most reported getting less than seven hours nightly. Today fewer teens get at least seven, let alone the nine their bodies need, than did teens in the 1990s.

Technology to blame? To an extent, yes; research shows all that screen time disturbs and delays sleep for all age groups who use them. Another commonly cited factor with some truth is increasingly competitive academics and college admission processes.

But the most surprising result of this study which differed from previous research was another factor: obesity. The spike in U.S. children's and teens' weight during the late 1990s corresponded with the biggest drop in sleep. Other factors have their negative impacts on sleep, say experts, but according to statistical correlations the worst in 20 years has been obesity, which disturbs sleep with its broad impacts on health.

Biologically, adolescents need slightly fewer hours of sleep than when they're younger, but more than adults 23 and older. Those aged 13 to 22 need nine to 10 hours of sleep each night. Hormonal changes during this period conspire against sleep. The body's production of sleep-inducing melatonin is delayed, making an early bedtime biologically impossible for most teens and young adults. Meanwhile, after-school activities now so necessary for competitive college admissions, jobs, and earlier than doctor-recommended school start times keep teenagers "literally sleepwalking through life," according to one researcher in a 2006 study by the National Sleep Foundation. That study reported similar findings (obesity wasn't considered), as well as a low energy and feelings of depression in sleep-deprived adolescents.

Another disturbing trend uncovered by the latest study is that students in (a) lower income families, (b) with parents who had lower education levels, (c) who are female (is this because more girls stay up later to study?), and (d) belonging to ethnic or racial minorities were all more likely to report regularly getting less than seven hours of sleep than their white, higher income counterparts (most of whom still didn't get more than seven hours).

Parents are generally unaware of how bad things are, with 90 percent polled in the National Sleep Foundation study reporting they believe their teens get "adequate" sleep. Most were also unaware that "adequate" means at least nine hours. While it doesn't address obesity or too-early school start times, researchers nevertheless recommend limiting caffeine drinks and limiting access to screens (computers, TV, iPads, etc.) in students' bedrooms, especially at night.

The Columbia study is the most comprehensive to date and builds on additional research with similar findings since the 1990s from medical schools, universities and hospitals.

Sholeh Patrick is a columnist for the Hagadone News Network. Contact her at Sholeh@cdapress.com.