A new study from researchers at the University of Colorado draws a connection between adults with insomnia who also used marijuana regularly as teenagers.

“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, one of the authors of the study published in the journal Sleep. “Our study adds to that literature, showing for the first time that early use is associated with increased rates of insomnia later on.”

Researchers looked at marijuana usage and sleep patterns for more than 1,800 twins whose average age was 23 and who completed a survey that also included questions about mental health. One-third of respondents who used marijuana before age 18 ended up with insomnia as adults, compared with 20% of those who did not. The finding was true – to a lesser degree – for people who started using marijuana as adults.

Winiger said that a variety of factors could explain the connection, including biology, structural changes to teens’ developing brains from cannabis, or poor sleep habits related to substance use. People who sleep poorly might also be drawn to marijuana as a possible solution, and the researchers noted that marijuana appears to be effective as a sleep aid in some circumstances.

“It is possible that sleep problems could influence cannabis use, cannabis use could influence sleep problems, or common genetics could be responsible,” the authors wrote.