Many cannabis users swear that weed can help them get a good night's sleep, but clinical evidence suggests that the connection between pot and sleep is more nuanced than it seems. A new study recently published in the Sleep journal found a link between adolescent cannabis use and adult insomnia, further complicating the issue.

Researchers from the University of Colorado Boulder looked at data from the Colorado Twin Registry, which has been collecting research data on twins since 1968. From this database, researchers identified 1,882 young twins that had completed surveys about their mental health, sleep patterns, and cannabis use. The researchers adjusted their findings to account for depression, anxiety, and night shift work, which can all impair sleep.

The study reports that around a third of all subjects who began using cannabis regularly before the age of 18 experienced insomnia as adults. In comparison, only 20 percent of subjects who did not regularly smoke weed as teens ended up with adult insomnia. Ten percent of teen cannabis users experienced “short sleep,” or sleeping six or less hours every night, compared to only 5 percent of non-users. Subjects who began using weed after age 18 also had slightly higher rates of insomnia as young adults.

Twin studies can be especially useful for research because pairs of twins have similar genetic makeup and usually grow up in the same environment. The present study collected data on 472 pairs of identical twins, who are genetically 100 percent identical, and 304 fraternal pairs, who share 50 percent of their genetic makeup. From this data, the researchers concluded that genes linked to insomnia are also connected to early cannabis use.

The study identifies a correlation between sleep disorders and teen cannabis use, but does not prove that teen pot use causes late-onset insomnia. “It is possible that sleep problems could influence cannabis use, cannabis use could influence sleep problems, or common genetics could be responsible,” the authors explain. The study theorizes that chronic teen pot use may lead to the development of poor sleep habits, but it could also be that teens are deliberately using weed to help treat their existing insomnia.

The study authors theorize that early pot use could possibly alter the development of the body's endocannabinoid system, which regulates sleep along with many other bodily functions. “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,” said lead author Evan Winiger in a statement.

Other researchers have explored the link between cannabis and sleep, with mixed results. A study from last year found that sleep-aid sales decreased in states with access to legal cannabis, providing evidence that many people use cannabis as a sleep aid. A 2017 study found that nearly 40 percent of chronic cannabis users suffered from insomnia, however. A more recent Israeli study found that medical cannabis can help chronic pain patients get quality sleep — but only if used occasionally. Chronic use was again linked to increased insomnia.

“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,” Winiger explained. “Our study adds to that literature, showing for the first time that early use is associated with increased rates of insomnia later on.”

“The evidence in adults is quite mixed, and unfortunately we currently do not have evidence from randomized controlled trials with different strains and different doses,” said co-author Ken Wright, director of the CU Boulder Sleep and Chronology lab, in a statement. Wright added that the ongoing prohibition of cannabis makes it difficult for researchers to conduct controlled double-blind trials involving marijuana.

Although the evidence is not fully conclusive, Wright recommends that teenagers should not “utilize marijuana to promote their sleep. Anytime you are dealing with a developing brain, you need to be cautious.”