What does the pronoun "their" refer to in the excerpt "Sleep...

                                    Hard Lesson in Sleep for Teenagers

By Jane E. Brody         October 20, 2014 

      Few Americans these days get the hours of sleep optimal for their age, but experts agree that teenagers are more likely to fall short than anyone else.

      Researchers report that the average adolescent needs eight and a half to nine and a half hours of sleep each night. However, in a poll taken in 2006 by the National Sleep Foundation, less than 20 percent reported getting that much rest on school nights. With the profusion of personal electronics, the current percentage is believed to be even worse. A study in Fairfax, Va., found that only 6 percent of children in the 10th grade and only 3 percent in the 12th grade get the recommended amount of sleep. Two in three teens were found to be severely sleep-deprived, losing two or more hours of sleep every night. The causes can be biological, behavioral or environmental. The effect on the well-being of adolescents — on their health and academic potential — can be profound. 

      Insufficient sleep in adolescence increases the risks of high blood pressure and heart disease, Type 2 diabetes and obesity, said Dr. Owens, pediatric sleep specialist at Children's National Health System in Washington. Sleeplessness is also linked to risk-taking behavior, depression, suicidal ideation and car accidents. Insufficient sleep also impairs judgment, decision-making skills and the ability to curb impulses, which are "in a critical stage of development in adolescence," Dr. Owens said. With the current intense concern about raising academic achievement, it is worth noting that a study by Kyla Wahlstrom of 9,000 students in eight Minnesota public high schools showed that starting school a half-hour later resulted in an hour's more sleep a night and an increase in the students' grade point averages and standardized test scores.

      When children reach puberty, a shift in circadian rhythm makes it harder for them to fall asleep early enough to get the requisite number of hours and still make it to school on time. A teenager’s sleep-wake cycle can shift as much as two hours, making it difficult to fall asleep before 11 p.m. If school starts at 8 or 8:30, it is not possible to get enough sleep. Based on biological sleep needs, a teenager who goes to sleep at 11 p.m, should be getting up around 8 a.m.

      Adding to the adolescent shift in circadian rhythm are myriad electronic distractions that cut further into sleep time, like smartphones, iPods, computers and televisions. A stream of text messages, tweets, and postings on Facebook and Instagram keep many awake long into the night.

      Parents should consider instituting an electronic curfew and perhaps even forbid sleep-distracting devices in the bedroom, Dr . Owens said. Beyond the bedroom, many teenagers lead overscheduled lives that can lead to short nights.

      Also at risk are many teenagers from low-income and minority families, where overcrowding, excessive noise and safety concerns can make it difficult to get enough restful sleep, the academy statement said. Trying to compensate for sleep deprivation on weekends can further compromise an adolescent's sleep-wake cycle by inducing permanent jet lag. Sleeping late on weekends shifts their internal clock, making it even harder to get to sleep Sunday night and wake up on time for school Monday morning.

                                                          (Adapted and abridged from http://www.nytimes.com)