The human brain is a fascinating and intricate structure we may never fully understand. Scientists have been trying to explain dreams for thousands of years, and research has theorized dreams as a way to store memories, a way of coping with stress, and a way to help sort through complicated thoughts and feelings. Some people even believe they’re our subconscious trying to tell us something. Dreams are a normal part of healthy sleep; they’re important to mental health, physical health, and emotional well-being.

Dreams are basically hallucinations that occur during certain types of sleep. There are four different sleep phases—the first four happen when you move between light and deep sleep, while the last stage, REM (rapid eye movement) sleep, is when brain activity is at its highest and vivid dreams develop. According to Matthew Walker, author of Why We Sleep and professor of psychology at the University of California Berkeley, there is more activity in the visual, motor, emotional, and autobiographical memory regions of the brain during REM sleep.

One common theory about the purpose of dreams is that they help filter and get rid of unimportant memories and process complicated feelings and thoughts. However, scientists aren’t all on the same page. Some believe they’re just neurons firing during sleep, and others believe they’re something deeper like a means of coping with life stress or a way to prepare for challenges. Although we know dreams affect memory storage, we aren’t yet sure how or why.

According to researcher Dr. Rosalind Cartwright, professor emeritus of psychology at Rush University in Chicago, “Dreams are almost like having an internal therapist, because you associate through dreams to previous similar feelings, and you work through the emotions related to it so that it is reduced by morning.”

Although dreams occur during both REM and non-REM sleep, brain activity is heightened during the REM phase. Studies have suggested that REM sleep affects how accurately people can read emotions and process external stimuli. According to research at Rutgers University, the quality of sleep before a traumatic event plays a big factor in how the brain reacts to a scary situation—the more REM, the weaker the fear-related effect. Other studies have shown dreaming has effects on memory and mood, while poor-quality REM sleep links to conditions like Alzheimer’s disease and depression. However, Walker believes that other stages of sleep are also important and carry health benefits like regulating blood flow and blood glucose levels as well as clearing Alzheimer’s-related plaque from the brain.

We spend over two hours dreaming each night, according to the National Sleep Foundation. Surprisingly, dreaming may also help mitigate depression. In a study of recently divorced women with untreated clinical depression by Cartwright and colleagues, they found that patients who recalled dreams and integrated ex-relationships into their dreams scored better on tests based on mood the next morning. They were also more likely to recover from depression when compared to others who did not dream about their marriage or could not recall their dreams.

Scientists and sleep researchers are still finding the answer to why we dream. Nevertheless, getting a good night’s sleep is beneficial to our health, and dreaming is a natural process that allows us to process emotional issues.