Starting school early in the morning is making it hard for teens to get enough sleep.

It all comes down to biological factors. According to clinical sleep psychologist Ellie McGlinchey, bodies often begin to suppress melatonin ― the hormone associated with sleep ― during puberty. This can mess with teens’ internal clocks, which in turn alters their sleep schedule.

“Their hormones are actually changing to make them want to go to bed later and wake up later,” she explained.

When students wake up at 6:30 a.m. or earlier to make it to school on time, they physiologically feel like it’s the middle of the night, McGlinchey said.

“They now are in a fog, at least for the first few hours of school,” she said. “They’re not concentrating well. They’re maybe feeling a little more emotional.”

The U.S. Centers for Disease Control and Prevention recommends that high schools start no earlier than 8:30 a.m., but as of last fall fewer than 20 percent of high schools followed the guideline.

“Even just delaying school by half an hour will actually have a big impact on a teenager’s ability to perform academically,” McGlinchey said.

Watch the video above for more on the negative effects of early school start times.