School starts way too early. Because early rising times are out of whack with our circadian rhythms, they’re responsible for teenagers not getting enough sleep, and being tired in the morning. It turns out that teens aren’t just lazy layabouts after all.

One in three Canadian teens don’t get enough sleep, says the study from McGill University in Montreal, meaning they fail to meet national sleep requirements. This leads to poorer performance at school, and previous studies link a lack of sleep in teens with increased likelihood of depression, anxiety, and other health problems.

“Adolescents are fighting biology to get to school on time,” lead author and McGill postdoctoral student Geneviève Gariépy told the McGill Newsroom.

[Photo: Tim Macpherson/Getty Images]

The study looked at 29,635 kids between 10 and 18 years old, pulling data from the Canadian 2013/2014 “Health Behaviour in School-aged Children” study. Bedtimes and wake times were self-reported, as were tiredness levels. Gariépy found that even while averaging 8.36 hours of sleep on weekdays, only 69% of kids were getting enough sleep, and 60% reported feeling tired in the morning. And–surprising to no one–“students went to bed later, woke up later, and slept longer on weekends.”

But perhaps the most interesting part, from a general knowledge point of view at least, is Gariépy’s explanation of why teens stay up so late and lay in bed all morning whenever they can.

“The problem is that early school start times conflict with the natural circadian clock of teenagers,” Gariépy said. “As teenagers go through puberty, their circadian clock gets delayed by two to three hours. By the time they reach junior high, falling asleep before 11 p.m. becomes biologically difficult, and waking up before 8 a.m. is a struggle.”

According to the study, Canadian schools start anywhere between 8 a.m. to 9:30 a.m. The 2016 Canadian 24-Hour Movement Guidelines recommend 9-11 hours of sleep for 5- to 13-year-olds, and 8-10 hours for 14- to 17-year-olds. No wonder you can never get them out of bed.