Cannabis May Disrupt Sleep Without Users Noticing, Long-Term Study Finds

Millions of Americans reach for cannabis when they want to sleep better, but a new study from Northwestern Medicine suggests the drug may be working against them in ways they never notice. Researchers followed 1,266 participants over nearly 35 years, from young adulthood into middle age, tracking their cannabis use and then measuring their actual sleep with wrist-worn monitors. What they found was a disconnect: people who used cannabis—both those with decades of cumulative exposure and those using recently—showed measurably worse sleep on the devices, yet reported feeling fine about how they slept.

The study, published October 9 in JAMA Network Open, examined data from the Coronary Artery Risk Development in Young Adults study, a long-running project that has tracked Black and white adults across the United States since 1985. Among the participants studied, those with higher lifetime cannabis use spent a smaller fraction of the night actually asleep, experienced more fragmented sleep, and lay awake longer after initially falling asleep. Recent cannabis users showed the same pattern: lower sleep efficiency, more fragmentation, and longer stretches of wakefulness. They were also significantly more likely to sleep longer than eight hours per night—a finding that itself suggests disrupted sleep architecture.

Yet when researchers asked these same people how well they slept, cannabis use made no difference to their answers. People who used cannabis reported sleeping just as well as those who never touched it. Dr. Thanh-Huyen T. Vu, the study's corresponding author and a research associate professor of preventive medicine at Northwestern's Feinberg School of Medicine, called this gap "one of the most interesting findings." The wearable device was picking up real disruption in the night—the kind that fragments sleep and reduces its restorative power—but the users themselves were unaware it was happening.

The researchers controlled for factors known to affect sleep: age, sex, race, cigarette smoking, alcohol use, physical activity, depression, chronic health conditions, and use of other substances. Even after accounting for all of these, cannabis use remained associated with objectively worse sleep. The study did not distinguish between different ways of consuming cannabis—smoking versus edibles, for instance—and the researchers acknowledged that consumption patterns likely shifted over the decades participants were followed, a question they flagged for future investigation.

Dr. Mercedes R. Carnethon, chair of the department of preventive medicine at Feinberg, framed the finding plainly: "Our data do not support the belief that cannabis is helpful as a sleep aid." As cannabis becomes legal and more widely available across the country, she added, understanding its long-term effects on health behaviors like sleep has become urgent. The study does not prove that cannabis directly causes sleep problems—correlation is not causation—but it offers some of the strongest evidence yet that the common assumption about marijuana and sleep is wrong.

For people looking to actually improve their sleep, the research points toward older, simpler strategies. Kristen L. Knutson, a sleep medicine specialist and associate professor at Feinberg, recommends keeping a regular bedtime, making the bedroom cool and dark and quiet, and putting away phones and screens before bed. A calming routine—reading, meditating, or a warm shower—can help. If sleep problems persist or if someone snores loudly, talking with a doctor to rule out an underlying sleep disorder matters. None of these require a substance that may be quietly fragmenting the very sleep it promises to improve.