Insomniacs are cozying up to cannabis. In the United States, where approximately 60 million Americans suffer from sleep disorders, many have started favoring flower or CBD tinctures to help usher in the sandman. And with reason. Research has demonstrated that cannabis can reduce the amount of time it takes to fall asleep, and can promote relaxation and reduce pain, which may also help ease the onset of sleep.

Cannabis, or more specifically, the cannabinoid THC, significantly affects two critical stages of sleep. Sleep consists of five stages of sleep in total: stages 1 through 4,  which is generally non-dreaming sleep, and the fifth, rapid eye movement (REM) sleep when dreaming takes place. In an ideal eight-hour night of sleep, we cycle through the five stages of sleep about three times on average.

From the limited studies that exist, weed appears to lengthen stage 4 sleep, the deepest stage of the sleep cycle characterized by the slowest brain-waves. That’s the stage that’s hardest to wake up from, and may explain why some users feel groggy upon waking. From stage 4, the brain backflips into REM sleep, where brain waves assume a highly active state, similar to the awake state. If you’ve had a toke before bed though, your brain may suppress the REM stage significantly.

“In some ways, cannabis impacts sleep in a manner similar to alcohol. It could help one relax and improve sleep onset, but then appears to disrupt later sleep, especially REM sleep,” explains Dr. Rubin Naiman, a sleep and dreaming specialist and clinical assistant professor of medicine at the Arizona Center for Integrative Medicine. “We become more vulnerable to awakenings in REM sleep since the body and brain are more activated.”

Although the specific mechanisms aren’t clear, we do know that CB1 receptors (check this out for a refresher on the endocannabinoid system), which THC binds to are also linked to REM sleep. Scientists have determined that different dosages or concentrations of THC elicit distinctive effects on slumber, with one constant: THC results in a reduction in REM sleep. Even with regular use where tolerance is built up, weed appears to continue to exert a suppressive effect on REM sleep.

While we’re still unraveling the mysteries of sleeping and dreaming, recent research points to REM sleep constituting a critical phase of our nocturnal wellbeing. We spend up to a quarter of the night dreaming, with the REM phase helping to process waking-life experiences by processing and assimilating the influx of experiences, emotional memories, and information that we absorb in everyday life.

Naiman suggests that dream deprivation contributes to a host of health and wellbeing concerns such as depression, cognitive difficulty, memory problems, increased inflammation, and increased risk for obesity. However, for individuals living with conditions such as PTSD who experience night terrors or frequent disturbing dreams, diminished time in REM may be advantageous, at least temporarily.