Humans have been fascinated by dreams throughout history. Ancient civilizations regarded what we see in our sleep as powerful messages from deities or an essential way to make sense of waking life.

Even today, there are societies with deep-rooted “dream cultures,” said Rubin Naiman, a fellow with the American Academy of Sleep Medicine and clinical assistant professor of medicine at the Andrew Weil Center for Integrative Medicine at the University of Arizona. “They believe dreaming is as important as waking,” he said, challenging the “wake-centric” perspective the rest of us impose on the world.

While we can’t definitively say what role dreams play in our waking lives, we do know they are important. Science shows that brain scans light up with activity when people dream, and there are physical responses that seem hardwired to encourage it, including body systems that essentially render you unconscious, paralyze you, and then wipe your memory of the dreamlike state. Yet the question remains: to what end?

If our body is supposed to be resting and restoring, why does your brain serve up seemingly senseless scenarios like a mash-up of snakes, school, and someone you ran into last week? Here are some of the reasons why you dream and the things that happen in your waking life that can influence them.

What is a dream?

A healthy sleep cycle rotates through four stages, said Chelsie Rohrscheib, head sleep specialist and neuroscientist at Wesper, a digital sleep clinic that aims to help people manage sleep disorders and improve sleep quality. Stages one through three flow through a light, medium, and deep state of rest, with your body gradually relaxing more and more. You then enter into REM — or rapid eye movement — sleep, when your brain looks almost as active as it does when you’re awake.

Most people who wake up during REM sleep will report having dreams, but dreams can also occur in different stages of sleep as well. A few physical changes play out during this part of our sleep cycle as well, Naiman said. The body and mind seem to separate.

First off, measures like your heart rate, breathing rate, and blood pressure start bouncing all over the place, looking more like what happens when we are awake. “We also experience something called REM atonia, which is fundamentally a paralysis of our voluntary muscles,” he adds. “We actually can’t move, and this makes sense — mother nature wants to protect us from acting the dream out.”

In fact, a (very) small percentage of the population has REM sleep behavior disorder, or a breakdown of this REM atonia process. According to research published in Frontiers in Neurology, people with the condition usually find out after winding up at the doctor’s office from injuring themselves or their partner “due to violent movements during sleep.” (There’s also something called sleep paralysis, where atonia persists even after you are conscious, temporarily making it impossible to move or speak.)

Then there’s the brain during REM sleep. While it starts powering up to resemble wakefulness, there are some key differences. “There’s a profound disconnect between the prefrontal cortex — our executive function — and what goes on in the lower limbic areas, particularly the hippocampus,” Naiman said.

In a nutshell: The parts of your brain responsible for logic and reasoning stop communicating with the parts involved in emotion, creativity, and memory.

Why don’t you remember your dreams?

This intra-brain disconnect is a big reason behind why your dreams often make no sense. With our logic centers more or less switched off, unrealistic scenarios like having the ability to fly feel real, Rohrscheib said. It’s also why you might struggle to remember dreams.

“We don’t actually activate our memory centers when we’re in REM sleep, because the brain is actively working on those centers,” Rohrscheib said. “So it kind of shuts down the process of forming new memories while you’re dreaming.”

If you do remember your dream on waking — even if it’s very fleeting — that just means you probably woke up in the middle of this REM stage.

So, why do we dream?

Science can’t exactly prove ancient civilizations wrong, but our understanding of why we dream has evolved. Sigmund Freud’s psychoanalytic approach in the early 20th century regarded dreams as messages from our own subconscious — reflections of our deepest desires and unfulfilled wishes.

Then there was the activation-synthesis hypothesis. Developed in the 1970s, this theory posits that dreams are nothing more than random series of images created by our brain’s REM activity. Any symbolism we impose on them is simply subjective.

“So we have these two extremes. One is deeply psychological, and the other is deeply biological,” Naiman said. While scientists still aren’t certain why you dream, today’s leading theories fall more in the middle of the spectrum.

You dream to form long-term memories

Each day is full of new experiences and information to process, much of which gets temporarily stored in our short-term memory, Rohrscheib said. Then overnight, the transfer of this short-term information into long-term memory requires your brain’s neurons to “replay” the initial experience, a process called consolidation. Much of this occurs during REM sleep.

But this sequence of neurons firing doesn’t necessarily follow a narrative or storyline — it can be totally random and disorganized from our perspective. “And that can result in weird dreams,” she said.

But it’s clear that dreams do play an important role in learning and memory, even if they don’t seem to make sense. Research shows, for example, that people learning a new physical skill, like playing tennis, perform better after sleeping compared with a similar amount of time spent awake.

The research also suggests that if you’re studying for a big test, you’re more likely to remember the information if you memorize it and then get some sleep, rather than pull an all-nighter.

You dream to process emotions

“There’s a bit of a consensus emerging that dreams are there to deal with emotionally challenging or difficult issues that a person is grappling with in their lives,” said Alan Eiser, a clinical lecturer at University of Michigan Medical School and faculty at the Michigan Sleep Disorders Center.

Naiman said to think of it like this: just like our gastrointestinal system is responsible for making critical decisions about what it allows in the body, REM sleep is like our brain’s gut.

“If you eat something that’s difficult to digest, it takes a little more effort on the part of the gut,” he said. “Likewise, experiences that are difficult to digest when we’re awake get processed symbolically in a dream state.”

This is why dreaming — not just sleeping — may actually be essential to our mental health. “We’ve known for 50 or 60 years that damaged dream patterns are associated with clinical depression,” Naiman said. “If we don’t dream, we suffer from a sort of psychological indigestion, which can show up as mood disorders.”

Research published in Current Biology points to a theory behind this emotional digestion. The study shows that during REM sleep, neurotransmitters associated with stress and anxiety in our waking life start to plummet. In the absence of these chemicals, our brain may be better equipped to process highly emotional or difficult experiences.

Dreaming may help you manage your stress response

“We also think [REM sleep] may be priming the area of your brain that responds to stress,” Rohrscheib said. She said to think of it like a full brain reboot after your neurons have been using energy all day to make connections. That’s why if you don’t get a sufficient amount of sleep, things that wouldn’t normally bother you suddenly set you off.

Dreams may even act as a sort of training exercise to deal with stress. A study published in the Journal of Sleep Research suggests that during REM sleep, you have high levels of activity in the amygdala (the part of the brain associated with our fight-or-flight reaction). While research is ongoing, this activity could be the brain’s way of prepping you emotionally to respond to stresses in waking life.

What affects dreams?

There’s still a lot we don’t know about dreams.

For instance: “We don’t understand why certain dreams are super common in the general population,” Rohrscheib said, such as dreams many people have experienced, like having your teeth fall out, flying, or being naked at school. “Why are these so similar across a vast variety of demographics?”

But researchers have homed in on some ways that behavior influences the brain’s REM sleep — and, therefore, may impact your dreams (for better and for worse).

Medication

“Certain antidepressants — like SSRIs and SNRIs — tend to suppress REM sleep early in the night,” Eiser said. “And then, late in the night, you get very dense REM periods.” That means there’s intense rapid eye movement happening and very vivid dreaming.

SSRIs, or selective serotonin reuptake inhibitors, include drugs like fluoxetine (Prozac) or sertraline (Zoloft), and SNRIs are serotonin and norepinephrine reuptake inhibitors, which include venlafaxine (Effexor) and duloxetine (Cymbalta).

But a laundry list of prescriptions list vivid dreaming or nightmares as potential side effects, including blood pressure medication, antihistamines, steroids like prednisone, cholesterol-lowering statins, the anesthetic ketamine, and drugs used to treat Parkinson’s disease.

Health conditions

Whether you got into a fight with your partner, have a big deadline coming up, or are nervous about a first date, everyone has bad or anxious dreams from time to time.

But you shouldn’t have consistent, chronic nightmares, Dr. Rohrscheib said. “It may be tied to your mental health, but other sleep disorders can cause nightmares, like narcolepsy or sleep apnea.”

People with anxiety or depression tend to be more likely to report having nightmares or bad dreams. A 2018 study in the journal Scientific Reports found that people who had more peace of mind in their waking life also had dreams to match, reporting more positive dreams where they felt amused or inspired. In comparison, people with anxiety tended to have more dreams with negative emotions such as fear or upset.

Nightmares are a symptom of post-traumatic stress disorder and often involve reliving the traumatic event. They are also a defining symptom of night terrors, which is when people (most often children) wake up screaming in intense fear but usually don’t remember the episodes in the morning.

About 20% to 30% of children have frequent nightmares compared with only 5% to 8% of adults.

Stress

“Stress affects the areas of the brain that are highly active during REM sleep,” Rohrscheib said.

Scientists believe that during REM sleep, these areas work to condition our brains so that we’re better able to cope with stressful situations during the day. So if you add more stress (or an anxiety disorder, for example), it could result in more stressful or bad dreams than usual.

Not enough sleep

If you’ve pulled a few all-nighters recently, research shows your brain actually tries to “catch up” on its REM cycles. It’s called the REM rebound effect and refers to how the frequency, depth, and intensity of REM sleep increases when we’ve been sleep deprived — all of which can trigger much more vivid, wild dreams.

Too much sleep

On the flip side, overindulging your snooze button could weigh down your dreams. A study published in Social Psychiatry and Psychiatric Epidemiology explored why people have nightmares.

While the team homed in on causes you might expect, like having anxiety or feeling negative emotions, they found that people who slept more than nine hours a night also reported more frequent nightmares.

Exercise

Some research suggests that regular exercise might lower the risk of nightmares and even dreaming in general by reducing both depression and time in REM sleep while increasing the amount of time spent in deep, restorative sleep.

But like most things in life, balance is key. A recent study published in Frontiers in Psychology suggests that people with either extremely low or extremely high activity levels can experience poor sleep quality.

Your period

If you get a period, you can thank your hormones for odder-than-usual dreams in the premenstrual phase of your cycle.

Research published in the Journal of Sleep Medicine and Disorders suggests that hormone fluctuations often alter sleep patterns — especially REM. While this can mean less time in dreamland, such disturbances can also wake you up more frequently, so you remember your dreams more vividly.

Still, these disturbances may come with a silver lining. A study published in Medical Sciences found that most people experience more pleasant dreams as their hormones bounce around before their period. And if you take a hormonal contraceptive, you may be even more likely to recall your dreams.

Alcohol

Having a nightcap may help lull you off to sleep, but alcohol actually suppresses REM sleep early in your slumber, which can lead to more extreme dreams later in your sleep cycle. “Then people tend to get a REM rebound with very vivid dreaming at the end of the night,” Eiser said.

Withdrawal from alcohol or drugs, like barbiturates and benzodiazepines, are associated with nightmares because of the REM rebound effect.

Cannabis

Marijuana users commonly report using the drug as a sleep aid, but consistent use actually suppresses dream states, Rohrscheib said, and recent research adds some context to this understanding.

A study published in Regional Anesthesia & Pain Medicine found that regular cannabis use was linked to unusual sleep patterns, with users more likely to say they were sleeping less than six hours a night or more than nine hours a night compared to nonusers.

Those longer and shorter sleep patterns could be linked to more vivid dreams, although it’s not clear if the cannabis use was the cause of the sleep patterns, the result, or unrelated. Because researchers suspect that cannabis suppresses our REM cycle, quitting may lead to some pretty strange, vivid dreams thanks to the rebound effect.

Sleep hygiene

Good sleep hygiene promotes good dream hygiene, according to the experts. In other words, doing those things that help promote sleep — like making sure you’re in a quiet, dark room and making other sleep-promoting lifestyle changes — can help dreams too.

“The goal is to get a sufficient amount of each sleep stage,” Rohrscheib said. “But if you don’t follow good sleep hygiene, you can, unfortunately, limit your brain’s ability to move through those sleep stages efficiently.” This means you might experience more sleep fragmentation — aka nighttime awakenings.“This can result in undercutting the amount of deep sleep and REM sleep you’re getting.”

So how do you build dreamier sleep hygiene? Rohrscheib said to stick to a strict sleep schedule (even on the weekends!), avoid things like alcohol, sugar, and caffeine before bed, and don’t lie in bed staring at a screen — the blue light it emits can keep your body from preparing for sleep. While there’s still a lot that we don’t know about dreams and what influences them, one thing seems certain: They’re important to our well-being.

“REM is the least understood sleep stage; we can only hypothesize why dreams happen,” Rohrscheib said. “But without REM sleep, we know it’s difficult for us to learn, to remember things, and we can even start to have issues with our stress response and overall mental health.”

By: Leslie Finlay