A team of researchers at The University of Texas at Dallas has found that long-term cannabis use is associated with difficulties with sleep, which might influence the severity of memory problems associated with use of the substance.

While previous research has examined how long-term cannabis use separately affects sleep and memory, the new study, published online June 25 in the American Journal of Drug and Alcohol Abuse, examined the interrelatedness of all three factors, said Dr. Francesca Filbey MS’97, professor of psychology in the School of Behavioral and Brain Sciences and the study’s corresponding author.

The results could better inform a variety of groups about the links between cannabis use and memory issues: patients who use medical marijuana, physicians who prescribe it and society in general.

“This work is critical for two reasons,” said Filbey, who is also the Bert Moore Chair in BrainHealth at the Center for BrainHealth. “First, so many questions remain unanswered about the long-term effects of cannabis use. Second, more Americans are gaining legal access to cannabis — a number that is growing quickly — but how little we know about its effects remains the same.”

About 1 in 8 adults in the U.S. use cannabis products, a portion that is increasing as cannabis legalization becomes more prevalent. About 10% of people who begin smoking cannabis are at risk, either through genetic or environmental mechanisms, for addiction. The diagnosis of cannabis use disorder (CUD) is characterized by problems related to cannabis use and includes symptoms that lead to negative health impacts.

“Our findings indicate that while there doesn’t appear to be a direct effect of long-term cannabis use on memory, the individuals with [cannabis use disorder] who reported more problems with sleep were the ones having poorer memory outcomes.”

Tracy W. Brown, a UT Dallas psychology doctoral student and lead author of the study

“It is crucial for those who have risk factors to understand the vulnerability associated with cannabis use,” Filbey said. “Effects can be long term, especially if you are younger than 25, and your brain is still maturing.”

The dual-site study with colleagues from the University of Amsterdam tested whether poorer learning and memory performance in individuals with CUD are due to cannabis-related sleep problems. One hundred forty-one adults diagnosed with CUD and 87 noncurrent users participated in the study. To avoid acute effects, no participants had used cannabis within the previous 24 hours.

“We’re trying to determine at what point cannabis use may lead to unintended consequences,” said Tracy W. Brown, a psychology doctoral student in Filbey’s Neuroimaging of Reward Dynamics Lab and the lead author of the study.

“Our findings indicate that while there doesn’t appear to be a direct effect of long-term cannabis use on memory, the individuals with CUD who reported more problems with sleep were the ones having poorer memory outcomes,” Brown said. “We’re discovering that cannabis’s indirect effect on cognitive outcomes is tied to how cannabis impacts the regulation of the sleep cycle.”

Unexpectedly, the researchers found that the impact of sleep and CUD on memory performance was more pronounced on spatial memory — which helps individuals navigate and remember where items are located — but did not affect verbal memory. Brown said that this might be due to the nature of the sleep-cycle disturbance induced by cannabis use.

“Typically, long-term cannabis use impacts slow-wave sleep which, outside the context of cannabis, affects hippocampal function,” Brown said. “Sleep disruption from cannabis use isn’t necessarily insomnia-related or complete sleep deprivation; rather, it’s a reduction in the quality of sleep. Full sleep deprivation is when you typically see verbal memory deficits.”

Filbey said the differences between subjects tested in Texas and the Netherlands — the study’s two sites — reflect the difficulty in coming to reliable conclusions about the effects of cannabis. In the Netherlands, personal use is decriminalized, and laws regarding its sale in certain venues are unenforced.

“Even though the Dutch overall use more cannabis, there’s greater CUD in Texas than in the Netherlands,” Filbey said. “This hints at how much cultural attitudes and lack of understanding of the drug contribute to a willingness in Americans to increase use.”

Filbey said adverse, irreversible effects in both sleep and in brain connectivity are of particular concern among users before age 25.

“Acute cannabis use affects many things, including sleep and working memory, and the field is undecided on whether those memory issues resolve after continued abstinence,” she said. “When use begins in adolescence, sleep deficits are usually much more pronounced. People who become dependent on cannabis for sleep risk developing a tolerance and requiring greater doses for the same effect. People need to be aware of this.”

The research was supported by a grant (1R01DA042490) from the National Institute on Drug Abuse, a component of the National Institutes of Health. Other authors are Dr. Robert Ackerman, associate professor of psychology at UT Dallas, and researchers affiliated with Erasmus University Rotterdam and the University of Amsterdam.