Although it’s not essential for sleep, “you definitely sleep better when your brain secretes melatonin appropriately,” Dr. Raj Dasgupta, a pulmonary, critical care, and sleep medicine specialist at the University of Southern California, tells Fortune. 

But what are melatonin supplements actually doing to our bodies and brains?

We push up our body block 

Melatonin is naturally released from the pineal gland, located in the middle of the brain and helps regulate the circadian rhythm, or the body’s internal clock. When adults take a melatonin supplement, potentially to combat jet lag or a night shift, it can signal to the body that it’s time for rest at a new time.

“What you’re doing is trying to give your body that hormone that’s naturally released anyway at a different time, to help you shift your circadian rhythm,” Dasgupta says. 

Melatonin may help adults fall asleep faster, and therefore increase duration of sleep and potentially sleep quality, or the ability to fall into REM or deep sleep.  

We may be taking too much

The FDA classifies melatonin as a dietary supplement for adults so there aren’t specific regulations on how much to take or for how long. Sleep scientists generally recommend between one and five milligrams for adults, yet not all brands of melatonin sold over the counter have clear dosage markers, and some may even be higher than what the label says. Taking too much melatonin can cause headaches, vomiting, and even changes in blood pressure.

We may have side effects

The hormone naturally occurs in our body, which is why there are few warnings about the side effects of the supplement, Dasgupta says. 

Still, people may feel daytime sleepiness, headache, dizziness, and upset stomach, he says, noting that if the symptoms persist or are more than mild to see a health care provider. 

Short-term melatonin use in adults is generally harmless, but there’s limited guidance when it comes to long-term use. After one to two months of routine use, experts recommend weaning off it to see how you fare without the supplement and whether you need further sleep interventions, Dasgupta says.

We can become psychologically dependent

Experts recommend cognitive behavioral therapy and a healthy wind-down routine (limiting screens before bed and being in dark, cool spaces) for treating insomnia. Melatonin alone “is not a very potent sleep aid” for adults, Dasgupta says, leading some to speculate that any positive results from melatonin are a placebo effect. 

So if we perceive that the melatonin supplement aids our sleep quality, we can become enticed to keep taking it—becoming “psychologically dependent” on it. Unlike an addiction, this mental draw to melatonin doesn’t cause the same harm that other drugs might, because the supplement doesn’t “cause the euphoria or pleasure associated with substances [known for] getting high,” Dr. Lewis S. Nelson, director of the division of medical toxicology and addiction medicine at Rutgers New Jersey Medical School, previously told Fortune. However, someone who is psychologically dependent on it may feel anxious if they can’t access it.

We may assume melatonin causes our nightmares 

Some also associate melatonin with increased nightmares or dreaming, especially closer to the morning. Melatonin may increase the duration of sleep, and therefore “indirectly” increase the amount of time you spend in REM, or deep sleep, which is when we have dreams and nightmares, Dasgupta says.

“By having more vivid dreams, it might make you feel like melatonin is the direct cause of those dreams,” he says. 

Sleep duration and quality is imperative, and melatonin use in the short term may help adults in what Dasgupta deems a “time-limited trial,” alongside other sleep regimens.