Sleep expert Michael J Breus, PhD, writes about teenage sleep patterns for Psychology Today.

Biologically, teens are programmed to be up and alert later at night and less awake in the morning. That puts their natural, biological sleep at odds with social time—particularly on school days. A teen who has to get up at 6:30 a.m. for school is like an adult who needs to get up for work at 4:30 a.m.

This biological-social clash for teens puts them especially at risk for sleep deprivation. During the school week, teens may lose as much as 2-3 hours of sleep a night. Biological clocks typically begin a shift back to earlier timing around the age of 20. But the years of adolescence can bring prolonged sleep deprivation during a critical time of physical, social, emotional, and intellectual development. During teenage years, the brain continues to undergo significant development—and sleep is essential fuel for that developmental growth.

Teen sleep amounts have been on the decline for decades. A first-of-its-kind, nationally representative study of teen sleep in the US found that teenagers’ sleep dropped significantly over a 21-year period from 1991-2012, with only about half of teenagers reporting sleeping 7 hours a night or more. The study also found that older teens reported sleeping less than younger teens, and that girls were more likely to report sleeping less than 7 hours a night than boys.