Introduction

Sleep plays an important role in maintaining both physical health and mental well-being. It is made up of several distinct stages, each serving a different function in the body's recovery process.¹ One of the most interesting and significant stages is Rapid Eye Movement (REM), during which most dreaming takes place; here the brain plays a key role in consolidating and processing memories and emotional responses.² In recent years, the use and role of cannabinoids (compounds derived from the cannabis plant, or endobiotics produced in the body) has generated interest regarding the role they play in sleep. Cannabinoids, marijuana, and marijuana-related products are often utilised to create a relaxing state and assist with the onset of sleep. Scientific research has demonstrated that cannabinoids produce significant changes in sleep patterns and in the REM stage of sleep.³

Understanding cannabinoids

Cannabinoids are chemicals that interact with specific receptors in the human body, which then modify processes related to mood, pain, appetite, and sleep. There are three main groups into which cannabinoids can be classified:⁴

• Endocannabinoids: neurotransmitters that are naturally occurring in the body
• Phytocannabinoids: naturally occurring compounds produced by the plant Cannabis sativa
• Synthetic cannabinoids: lab-produced substances for medical and research purposes

Two phytocannabinoids of note are tetrahydrocannabinol (more commonly known as THC), which produces the psychoactive effects or 'high' that people experience with cannabis,⁵ and cannabidiol (CBD), which does not have psychoactive properties and is commonly used for its calming or anti-anxiety effects and in the management of epilepsy.⁶

The body's endocannabinoid system (ECS) helps regulate many internal processes and maintain balance (homeostasis). The ECS contains cannabinoid receptors (CB1 and CB2) located throughout the brain and nervous system. CB1 receptors are selectively concentrated in areas of the brain that regulate sleep patterns and cycles, which may explain why cannabinoids can alter when and how we sleep, including how long we spend in REM sleep, after they activate receptors in the hypothalamus, amygdala, and brainstem.⁷

The normal REM sleep cycle

A normal sleep period consists of cycles of Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. In the first few stages of NREM sleep, progression occurs from light sleep to deep sleep. During REM sleep, when most dreaming takes place, brain activity is generally characterised as high and the body is temporarily paralysed by muscle atonia, preventing movement. A healthy adult typically cycles through NREM and REM sleep about four to six times each night. In the early part of the night, deep NREM sleep predominates, whereas later REM periods become longer and more frequent. REM sleep is extremely important for the regulation of emotions, memory consolidation, and cognitive performance. Disruption to REM sleep can result in mood changes, decreased concentration, and difficulty with memory.⁸

Effects of cannabinoids on REM sleep

One of the most well-documented effects of THC is its suppression of REM sleep. Some may find not dreaming vividly helpful, particularly those suffering from vivid nightmares or post-traumatic stress disorder (PTSD). However, it may also cause some individuals to feel less rested and more alert over longer periods of time.⁵ When someone stops THC use, the body often responds with a process known as 'REM rebound,' during which REM sleep increases in frequency and dreams become more vivid.² This indicates that THC use does disrupt the REM sleep cycle.

Unlike THC, CBD affects sleep differently, as it does not produce a 'high.' Its effects are often more subtle and calming. For patients suffering from chronic pain, anxiety, and epilepsy, CBD tends to increase total sleep time and sleep quality.⁶ Generally, CBD appears to promote a stabilising effect on sleep without the pronounced suppression experienced with THC.

Synthetic cannabinoids, which are considerably stronger than those found in nature, are engineered to produce more potent yet less predictable effects. As their general mechanism of action is to overstimulate the body's cannabinoid receptors, some users may experience significant REM suppression and poor-quality, disrupted sleep. Side effects associated with withdrawal from synthetic cannabinoids may include vivid dreams or even insomnia, indicating that the effects on the sleep cycle can be profound and potentially dangerous.³

Mechanisms behind the effects

The reason cannabinoids affect the REM sleep cycle is that THC and other cannabinoids bind to CB1 receptors, impacting the release of various neurotransmitters such as GABA, serotonin, and acetylcholine, all of which are involved in regulating the sleep-wake cycle. This process alters the way the brain transitions through sleep stages, generally decreasing the overall time spent in REM during a sleep period.^3,4

Short-term vs. long-term impact

In the short term, cannabinoids can help individuals fall asleep more easily and for longer periods. For this reason, some people may consume cannabis before bed. However, this comes at the cost of reduced REM sleep, meaning less time for the body to emotionally and cognitively recover.²

Individuals who use cannabis long-term, particularly high-THC products, may experience altered effects over time. As the brain becomes accustomed to receiving cannabinoids more frequently, it may become difficult to maintain normal sleep cycles without the compound. Upon cessation, users may experience vivid dreams, poor sleep quality, and restlessness — classic symptoms of REM rebound. Ultimately, long-term use may assist with sleep initiation temporarily, but it is likely to disrupt deeper, healthier sleep cycles.^4,5

Clinical and therapeutic considerations

Despite the drawbacks and side effects associated with cannabinoid use, there is potential therapeutic value for a limited number of sleep-related disorders. For example, individuals with PTSD often experience distressing nightmares resulting from excessive REM cycles. THC can suppress REM sleep, which may reduce the frequency or intensity of nightmares. However, long-term suppression of REM sleep may interfere with emotional processing and create future difficulties.^4,5

The use of CBD may reduce anxiety and contribute to reducing inflammation, which may improve sleep quality while minimising effects on REM sleep. However, current evidence is derived from small studies, and results vary depending on dosage, administration method, and individual differences.^4,6

Summary

While cannabinoids clearly have an impact on REM sleep, the relationship between cannabinoids and sleep is not straightforward. THC generally decreases REM sleep and dreaming, whereas CBD appears to have a more neutral effect. Short-term use may assist users in falling asleep faster and may help with nightmares or other sleep-related issues. However, long-term reliance on cannabinoids for sleep typically leads to tolerance and alterations in the sleep cycle, including reduced REM sleep or REM rebound upon cessation. Cannabinoids may be a valid consideration for therapeutic use in conditions such as insomnia or PTSD, though caution is warranted. Sleep is a delicate and vital process, and understanding the relationship between cannabinoids and sleep is crucial to ensuring safe use — maximising positive effects while minimising risks.