Feeling like you slept well isn’t the same as sleeping well.

Researchers at Northwestern University drew on about 35 years of data from 1 266 US adults. The more cannabis people had used over their lives, the more broken and less efficient their sleep looked on a wrist monitor. Yet they didn’t report sleeping any worse.

If you use cannabis to sleep, that gap is worth knowing about.

The study tracked cannabis use for 35 years

Published in JAMA Network Open on October 9, the study used CARDIA, a long-running US project that began in 1985–86 and has followed Black and white adults from young adulthood into middle age.

Participants were repeatedly asked about cannabis over the years. That let the researchers measure two things:

  • Cumulative use, in “cannabis-years”. One cannabis-year equals 365 days of use, spread across any part of your life.
  • Recent use: any use in the past 30 days.

In middle age, sleep was measured two ways. A wrist monitor, worn for seven days, estimated sleep and wake from movement. A standard questionnaire, the Pittsburgh Sleep Quality Index, asked people how they thought they had slept.

891 people had monitor data, with an average age of about 61. 1 128 completed the questionnaire.

Each cannabis-year was linked to more broken nights

The researchers accounted for age, sex, race, depression, physical activity, alcohol, smoking, medications and other substance use. Each extra cannabis-year was still linked to higher odds of:

  • 10%: low sleep efficiency, meaning less than 85% of time in bed spent asleep.
  • 4%: highly fragmented, restless sleep.
  • 21%: lying awake for 30 minutes or more after first falling asleep.

Those are per cannabis-year, so they add up with heavier lifetime use.

Recent users looked worse still

Compared with people who had never used cannabis, recent users had about three times the odds of long night-time wakefulness. They were also more likely to have low sleep efficiency and fragmented sleep.

And they were more likely to sleep more than eight hours.

More time in bed, broken up by more time awake.

People’s own sleep ratings didn’t pick up the disruption

Here’s the twist. On the questionnaire, cannabis use was not linked to reporting poor sleep.

The monitors saw more disrupted nights. The sleepers didn’t.

As the study’s senior author, Mercedes Carnethon, put it: “Our data do not support the belief that cannabis is helpful as a sleep aid.”

You can’t always feel how badly you slept

Sleep has several sides: how long it lasts, whether it’s unbroken, and how rested you feel. They don’t always line up.

Brief awakenings are often forgotten by morning. You can spend eight hours in bed, wake many times, and still feel you slept reasonably well.

Your memory of the night is not a recording of it.

If something makes you relaxed or drowsy, your verdict on the night may be kinder than the night was. To be precise, the study doesn’t show that users believed cannabis improved their sleep. It shows that their sense of how they slept didn’t reflect what the monitors recorded.

What the study can’t tell you

  • It’s a peer-reviewed observational study. It can’t show that cannabis causes poor sleep.
  • The arrow may partly point the other way: people who already sleep badly may be more likely to try cannabis. Other differences between users and non-users may remain despite the adjustments.
  • It didn’t separate smoking from edibles, or different products and doses. People’s habits also likely changed over 35 years.
  • Wrist monitors estimate sleep from movement. They’re less precise than a lab sleep study, which measures brain activity.
  • It didn’t test whether stopping cannabis improves sleep.
  • The advice below is a reasonable inference, not something the study tested.

Don’t trust the “it helps me sleep” feeling

Feeling rested is not proof that you slept well.

If you rely on cannabis at night and still struggle with sleep, ask a doctor about cognitive behavioral therapy for insomnia, which has strong evidence behind it.