Teens who don’t get enough sleep may be at risk for chronic problems later in life from increased inflammation throughout the body, according to a new study.

Those who didn’t get enough sleep during the week and especially those who slept longer on weekends had higher inflammation levels tied to heart disease and diabetes, researchers report in Sleep Medicine.

Investigators used blood tests to measure kids’ C-reactive protein (CRP) levels. High levels indicate that the body is in “inflammation mode,” said lead author Martica Hall of the psychiatry department at the University of Pittsburgh in Pennsylvania.

“Frankly, high CRP over a long period of time is bad for everything,” Hall said. “We say it increases the risk of ‘all things bad.’”

This study only measured CRP levels at one point in time, but “folks that have chronically high CRP are at risk for heart disease and diabetes,” Hall told Reuters Health by phone. “For adults we know that short sleep is associated with CRP levels.”

The researchers also had the 244 healthy high school students wear wrist monitors that recorded how much they slept.

Most kids had CRP levels in the low to moderate risk range, below 3 milligrams per liter, but 33 students qualified as “high risk” by scoring above that threshold.

On average, the kids got less than six hours of sleep per night during weekdays and between seven and eight hours on the weekends.

Teens who slept the least during the week were most likely to be in the high-risk group. Additionally, those who slept at least two hours longer on the weekends were more than twice as likely to be in the high-risk group, compared to those who had more consistent sleep times throughout the week.

Kids in the high-risk group also tended to have a higher body mass index, which is a measure of weight in relation to height.

Sleep duration was still linked to CRP levels even when the authors accounted for depression, sex, race, parental education and body mass index, which is usually an important predictor of CRP levels, Hall said.

“The kids who have a big discrepancy between the amount of sleep they get between weekdays and weekends, as well as those who have short sleep during the school day, that’s associated with higher level of CRP,” Hall said.

Similar results have been seen in adults and the elderly, but these studies that look at one moment in time can’t prove cause and effect, Dr. Francesco Cappuccio told Reuters Health in an email.

“In other words, is sleep deprivation causing an activation of inflammation or is an underlying reason for raised inflammatory markers the cause of reduced sleep?” said Cappuccio, a cardiovascular physician at the University of Warwick in Coventry, U.K. who was not involved with the new study.

Caution is always a good approach for results like these, but there are plenty of reasons for believing that sleep deprivation could be harmful, he said.

CRP levels can be lowered, but it would take a separate experiment to determine if changing sleep patterns would effect that change, Hall said.

“The cool thing about sleep is that it’s changeable,” she added.

Hall said kids and parents should focus on lengthening sleep times during the school week and not worry so much about the weekend. Other studies have found that highly variable sleep lengths are associated with health risks, so standardizing and lengthening sleep for five of the seven days of the week is a good start, she said.

“One of the reasons why there is such a move for trying to change high school start times is this is a population which is biologically predisposed to be awake at night,” Hall said.

In August, the American Academy of Pediatrics recommended schools delay start times to at least 8:30 a.m. to improve the physical and mental health of students.

“They are predisposed to be awake at night then have really early start times, and that may increase the risk for way downstream disease,” she said.