Marijuana users sleep measurably worse than people who never use it, despite the near-universal belief that a joint before bed guarantees a good night.
The finding comes from a study that followed 1,266 people for roughly 35 years, tracking them from young adulthood into middle age and monitoring their cannabis use along the way.
In middle age, participants wore wrist sleep monitors for a week and completed surveys about how well they thought they slept, letting scientists put perception and measurement side by side.
The gap was stark. Heavier lifetime cannabis exposure tracked with worse objectively measured sleep, including less time actually asleep as a share of the night and more disruption. Recent users showed more fragmented sleep, lower sleep efficiency and more time lying awake after initially dropping off. They were also far more likely than non-users to sleep beyond eight hours a night.
The most revealing wrinkle is what participants did not notice. Cannabis users rated their own sleep quality no worse than anyone else's, even as their wrist monitors told a different story. Someone can be fully convinced marijuana is knocking them out each night while the data shows the opposite.
Researchers accounted for age, race, sex, tobacco use, alcohol consumption and physical activity, and the link between cannabis and poorer sleep held up. They did not compare consumption methods such as smoking versus edibles, and more work is needed to pin down the long-term effects.