A third of people who smoked marijuana as a teenager later developed insomnia as adults, according to a new study.

About one in three Americans don’t get enough sleep, according to the Centers for Disease Control and Prevention (CDC). Problems like insomnia or sleep apnea can arise for multiple reasons, including stress, PTSD, and chronic pain, limiting your ability to achieve a good night’s rest. Many have turned to marijuana as an effective sleep aid, thanks to the plant’s ability to reduce inflammation and restore a person’s natural sleep cycle.

But a new study, published in the journal Sleep, suggests the root of insomnia can begin by smoking copious amounts of marijuana as a teenager. University of Colorado Boulder researchers analyzed data from more than 1,800 twins, as recorded by the Colorado Twin Registry. The organization has been tracking these sets of twins since 1968, but the study’s authors focused on data related to sleep, marijuana use, and mental health.

According to the study, about a third of the subjects who started smoking weed before the age of 18 had developed insomnia in adulthood. Less than 20% of those who didn’t use cannabis as teens had similar sleep issues.

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“People tend to think that cannabis helps with sleep, but if you look closely at the studies, continued or excessive use is also associated with a lot of sleep deficits,” said Evan Winiger, the study’s lead author and CU graduate student.

Teenage marijuana users had double the risk of acquiring “short sleep” as adults. A hazardous variation of insomnia, “short sleep” constitutes getting less than six hours of sleep per night on a regular basis. Almost 10% of teenage marijuana users were victims of “short sleep” while only 5% of non-users had this issue.

Though the study did find a link between early cannabis use and adult sleeping issues, the authors didn’t pinpoint an underlying cause as to why. Smoking marijuana as a teenager is associated with negative long-term changes to brain structure, having mental health disorders like depression and anxiety later in life, as well as breathing problems.

Winiger pointed to alterations in the brain’s gray matter as possible explanation to the connection. But he also suggested the body’s endocannabinoid system could also be modified from the influx of marijuana at a young age.

“One theory is that these receptors are being desensitized or disturbed from all the cannabis use at a time that the brain is still developing, and that leads to waking issues later,” he said.

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But the study’s authors added this doesn’t mean all marijuana is bad for all people looking to fall asleep. Instead, the research indicates another sign of how using cannabis at a young age could negatively impact adults later in life.

“We would not recommend that teenagers utilize marijuana to promote their sleep,” said study co-author Ken Wright, director of CU’s Sleep and Chronobiology lab. “Anytime you are dealing with a developing brain you need to be cautious.”