Marijuana may be associated with impaired sleep quality, according to a recent study.

Researchers led by Jilesh Chheda, research assistant at the University of Pennsylvania in Philadelphia, found that any history of cannabis use was associated with an increased likelihood of reporting difficulty falling asleep, struggling to maintain sleep, experiencing non-restorative sleep, and feeling daytime sleepiness.

They found that the strongest association was found in adults who started marijuana use before age 15; they were about twice as likely to have severe problems falling asleep (odds ratio = 2.28), experiencing non-restorative sleep (OR = 2.25) and feeling overly sleepy during the day (OR = 1.99). Results were adjusted for potential confounders including age, sex, race/ethnicity and education, according to a press release.

"Current and past marijuana users are more likely to experience sleep problems," Chheda said in a statement. "The most surprising finding was that there was a strong relationship with age of first use, no matter how often people were currently using marijuana. People who started using early were more likely to have sleep problems as an adult." 

For the study, researchers collected data from more than 1, 800 adults ranging in age from 20 to 59 years old who responded to the 2007-2008 National Health and Nutrition Examination Survey (NHANES). Cannabis use was assessed as any history of use, age at first use and number of times used in the past month. Sleep-related problems were considered severe if they occurred at least 15 days per month.

What the researchers found suggests that initiation of marijuana use in adolescence may impart a higher risk for subsequent insomnia symptoms, or it may mean that those who begin using earlier are more likely to experience insomnia for other reasons, such as stress. Insomnia may even be one of the reasons people start (or continue) use, though this evidence suggests that it is probably not effective if they are still experiencing problems.

"Marijuana use is common, with about half of adults having reported using it at some point in their life," Chheda said.  "As it becomes legal in many states, it will be important to understand the impact of marijuana use on public health, as its impact on sleep in the 'real world' is not well known."

The findings were recently published recently in an online supplement of the journal Sleep.