The Role of Melatonin in the Body
Melatonin is a hormone naturally produced by the pineal gland in the brain, primarily in response to darkness [1.5.2, 1.9.4]. Its main function is to regulate the body's internal clock, also known as the circadian rhythm, which manages the sleep-wake cycle [1.5.3]. As light fades in the evening, melatonin levels rise, signaling to the body that it's time to prepare for sleep by causing drowsiness and lowering core body temperature [1.5.2, 1.3.4]. Its production is inhibited by light, which is why exposure to screens before bed can disrupt sleep patterns [1.5.3]. Beyond its role as a sleep regulator, melatonin is also involved in immune function, mood regulation, and acts as a powerful antioxidant [1.5.2, 1.5.4].
Understanding the Stages of Sleep: NREM and REM
Sleep is not a uniform state; it is composed of several cycles that alternate between two main types: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. A typical night's sleep consists of several of these cycles.
- NREM Sleep: This is divided into three stages. Stage N1 is the light transition period into sleep. Stage N2 is a more stable sleep stage where the body prepares for deep sleep. Stage N3, also known as deep sleep or slow-wave sleep, is the most restorative stage, crucial for physical recovery and growth.
- REM Sleep: This stage is characterized by rapid eye movements, increased brain activity, and vivid dreaming [1.4.5]. It is vital for cognitive functions such as memory consolidation, learning, and emotional regulation [1.2.5]. During a typical night, REM sleep periods become longer in the latter half of the sleep episode [1.3.2].
Examining the Evidence: Does Melatonin Stop REM Sleep?
Contrary to the concern that it might suppress this vital sleep stage, most research indicates that melatonin does not stop or significantly reduce REM sleep [1.3.1, 1.4.6]. Instead, its primary influence is on the timing and onset of sleep. Low-dose melatonin has been shown to promote sleep onset without altering the overall sleep architecture [1.3.1].
Some studies have even found that melatonin can increase the percentage of REM sleep. One study on patients with reduced REM sleep duration found that a 3 mg dose of melatonin significantly increased REM sleep percentage [1.2.1]. Another study observed that melatonin increased REM sleep propensity, particularly in the first REM episode of the night [1.4.1].
However, the effects can be complex. One analysis noted that melatonin might increase REM sleep duration in the first half of the sleep opportunity and decrease it in the second half [1.4.2]. The impact can also vary based on dosage, timing of administration, and the individual's specific sleep disorder. For instance, in patients with Delayed Sleep Phase Syndrome, melatonin is effective at helping them fall asleep faster but may not affect the percentage of time spent in REM sleep [1.2.4]. A recent breakthrough study even identified the melatonin MT1 receptor as a crucial regulator for enhancing REM sleep, suggesting future therapeutic potential [1.2.5].
Melatonin vs. Other Sleep Aids: Impact on REM Sleep
Different sleep aids have markedly different effects on sleep architecture, especially on REM sleep.
| Sleep Aid Class | Effect on REM Sleep |
|---|---|
| Melatonin | Generally does not suppress REM sleep; some studies show a slight increase or no significant effect [1.3.1, 1.2.1]. Its main action is regulating the circadian rhythm [1.5.3]. |
| Benzodiazepines | Known to be REM sleep-suppressant medications. Withdrawal can lead to a phenomenon called "REM sleep rebound" [1.6.2]. They also decrease deep N3 sleep [1.6.4]. |
| Z-drugs (e.g., Zolpidem) | Generally have minimal effects on sleep stages at clinical doses, though high doses can reduce REM sleep [1.6.4]. They are more selective than benzodiazepines [1.6.6]. |
| Antidepressants | Many antidepressants are known to significantly decrease the amount of time spent in REM sleep [1.6.1]. |
Important Considerations for Use
The effects of melatonin can be influenced by several factors:
- Dosage: Low doses (under 1 mg) may be effective for advancing sleep phase, while higher doses (up to 5 mg) may be better for improving sleep consolidation [1.3.3]. Experts recommend starting with the lowest possible dose and not exceeding 10 mg per day [1.8.5].
- Timing: To advance the body's sleep clock, melatonin is most effective when taken in the evening [1.5.3].
- Quality: Melatonin is classified as a dietary supplement in the U.S. and is not regulated by the FDA [1.5.3]. Studies have found significant discrepancies between the labeled dose and the actual melatonin content in commercial products, with some even containing serotonin [1.9.5].
Common side effects of melatonin include drowsiness, headache, dizziness, and nausea [1.8.3]. Some people also report vivid dreams or nightmares [1.8.1]. It's recommended to consult a healthcare provider before starting melatonin, especially for long-term use or for individuals with underlying health conditions [1.9.4].
Conclusion: A Regulator, Not a Suppressor
The available scientific evidence indicates that melatonin's primary pharmacological function is to regulate the circadian rhythm, effectively signaling to the body when it's time to sleep. It does not "stop" or broadly suppress REM sleep. While its effects on sleep architecture can be complex and vary between individuals, many studies show it has either a neutral or even a slightly positive impact on REM sleep duration [1.2.1, 1.4.1]. Unlike other classes of hypnotics, such as benzodiazepines, which are known to suppress REM sleep, melatonin's main benefit lies in its ability to help align the sleep-wake cycle with the natural day-night rhythm [1.6.2, 1.5.3].
Authoritative Link: National Center for Complementary and Integrative Health - Melatonin
