Teenagers are not getting enough sleep, according to new research from the Mailman School of Public Health, managed by Columbia University. Now researchers want to know the causes for their lack of rest.  

Sleep habits of teenagers were measured over a period of two decades by researchers, who surveyed 272,077 people who went through their teen years between 1991 and 2012. Participants were asked to report how often they slept for seven hours or more a night during that period. Researchers found the amount of sleep reported by the subjects decreased over that period, suggesting teenagers are sleeping less than they were 25 years ago.

"Seven hours per night is two hours less than the nine hours recommended by the National Sleep Foundation, indicating a particularly concerning trend toward inadequate sleep for a large portion of U.S. adolescents at an important juncture in development," Julie Maslowsky of the University of Texas at Austin said.

A total of 24 percent of participants reported getting enough sleep in 2012, down from 30 percent in 1991. Around 63 percent of participants reported taking seven hours or more of sleep every night in 2012, compared to 72 percent in 1991.

Internet use at night, including research for homework and social media, could play a role in keeping teens awake, investigators believe. Rising obesity rates could also play a factor in this change, as the condition is commonly linked to sleep disorders. Even increased competition for admission to the colleges could play a role in the lack of sleep experienced by adolescents, researchers theorize.

Lack of sleep can also take its toll on students who cannot pay as much attention during class as they otherwise could if they were well-rested. This can have an impact on their academic performance, as well as health.

Females, teenagers who are members of racial minorities, and those in lower financial classes were found to be the groups most likely to suffer from sleep deprivation. Males, especially Caucasians, and those from wealthier families, are experiencing the smallest changes to their sleep patterns, the new study revealed. Oddly, although ethnic minorities and those from lower financial classes received less sleep than other participants, they reported receiving adequate sleep more often.

"This finding implies that minority and low socioeconomic status adolescents are less accurately judging the adequacy of the sleep they are getting," Katherine Keyes from the Mailman School of Public Health said.

Some school systems around the United States have been experimenting with later start times for the school day, which seem to be having a positive influence on study habits.

Analysis of sleep patterns in American teenagers was profiled in the journal Pediatrics.