What Are Dreams?

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The mystery of dreams has captivated people for millennia. Questions abound about their purpose and nature. Are they messages from the subconscious? Repressed desires? Random and meaningless firing of neurons in the brain?

Answers to these questions seemed elusive — until recently. The modern study of dreams began with the discovery of REM sleep in the early 1950’s. A few years afterward, research revealed that dreams were more vivid and memorable during REM sleep, and further studies showed that eye movements recorded during REM sleep matched dream imagery described by participants of sleep studies.


Most dreams appear to manifest the interests and views of each individual dreamer. Though the contents of dreams are subjective, with imagery that can’t be seen by researchers but only reported by study subjects, advances in sleep science are leading to a greater understanding of what happens while we dream. The answer to the question of why we dream may not be far off.

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.

What Are Dreams?

Dreams are a collection of involuntary thoughts, visual images, and emotional responses that occur during sleep. Sounds and physical sensations may also be experienced in dreams.

Once thought to occur only during the REM stage of sleep, dreams are now known to take place during the three non-REM phases of sleep, as well. (1)

Dreams appear to be triggered when the usual activity of the brain changes or decreases, as it does during sleep. Dreams are initiated when the hypothalamus signals areas of the brain responsible for wakefulness, cueing them to power down. Dreams begin in early childhood and increase in number and length until adulthood.

The average person dreams three to five times each night, though some particularly prolific dreamers may dream up to seven times in a single night. While most dreams last between five and twenty minutes, some last only seconds, and many are never remembered. Approximately six years of our lives are spent dreaming.

Paradoxical Sleep

Another term for REM sleep. REM is sometimes referred to as paradoxical sleep because brain activity during this stage so closely resembles that of wakefulness.

Dreams: REM Sleep Vs. Non-REM Sleep

Dreams may begin as quickly as a few seconds into the first stage of sleep. These types of dreams are typically disorganized snatches of thoughts, images, and sensory perceptions called hypnagogic hallucinations, and may be interspersed with brief shifts back to a waking state.

Once the body begins the transition to the sleep state, neurons in the brain fire erratically. Dreaming may occur when the part of the brain that processes neuronal signals attempts to make sense of the disorganized responses that occur during sleep.

Though dreaming takes place during all four stages of sleep, including the three non-REM phases of sleep, the nature of dreams may be quite different depending on the stage during which they occur. (2)

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Non-REM Dreams

During non-REM sleep, heart rate and blood pressure decrease, muscles may twitch, and brain waves switch from the alpha waves of the wake state to the theta waves of the sleep state. (3)

The three phases of non-REM sleep comprise approximately 75% of total sleep time, and can be described as follows:

  • N1. This is the lightest phase of sleep, the transition between the sleep and wake states
  • N2. A slightly deeper phase of sleep. Most of the time spent asleep is spent in N2 sleep
  • N3. Also known as slow wave sleep, N3 is the deepest and most restorative phase of sleep

EEG testing reveals consistent dreaming activity during non-REM sleep, though non-REM dreams appear to be less vivid with lower emotional content. Subjects in dream studies often have more difficulty recalling dreams that occur during one of the non-REM phases of sleep.

Though generally more disconnected, dreams that occur during non-REM sleep may be intimately connected to REM sleep. (4Non-REM dreams tend to occur during the early morning hours when REM sleep is more likely, and may be induced by activation of the same part of the brain that is aroused during REM sleep.


How do we know animals dream? EEG studies of the animal brain show that all mammals dream during REM sleep, and birds and reptiles may dream, as well.

REM Sleep Dreams

Most dreams occur during REM sleep, when brain activity most closely resembles that of being awake. In addition, REM sleep is distinguished by quick movements of the eyes and paralysis of the muscles. Heart rate increases compared to non-REM phases of sleep,

Brain activity during REM sleep shows mixed brain waves that closely resemble those seen during the wake state. (5) Similar to the waking brain, the brain during REM sleep displays less synchronous patterns and more random activity, including in regions of the brain that process sensory information.

REM sleep dreams may be more vivid than those that occur during non-REM sleep due to activation of the brain’s visual cortex. This activation may be central to the nature of dreams, in that they are “seen” by the brain just as any image is seen while the dreamer is awake.

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By contrast, the prefrontal cortex, the part of the brain responsible for logic, decision-making, and planning, shows reduced activity during REM as well as non-REM sleep. This inability to reason during dreams may be the reason that most people do not know they’re dreaming.

A person awakened during REM sleep is more likely to remember his or her dream. As evidenced on EEG, dreams that draw on the experiences of waking life tend to be associated with the theta wave activity of REM sleep, further bolstering the idea that REM sleep is associated with the processing of emotion and memory.

The Reticular Activating System

The reticular activating system — or RAS — controls both the sleep and wake states, as well as the fight or flight response. The RA aso controls the flow of information that we let into our conscious minds, helping us separate important sensory signals we need to pay attention to from less important information that can be ignored. (6)

The RAS is responsible not only for waking us up in the morning, but for activating the brain in general. (7The system ignores small sounds that might disturb us when we need to sleep, but pays attention to them when it’s time to wake. The activation of the RAS may be the reason we incorporate certain sounds and sensations into dreams just before waking, when arousal of the brain and sensory processing systems begins.

Electroencephalogram, or EEG

A test that measures electrical activity in the brain. It is used to diagnose epilepsy, sleep disorders, and other conditions that impact brain waves and neuronal activity.

Oneirology: The Scientific Study of Dreams

The word oneirology comes from the Greek, and means the study of dreams. Though this may sound like the study of what dreams mean, oneirologists do not try to interpret dreams or understand their meaning. Rather, they study the physiological process of dreaming.

Oneirology involves the search for correlations between brain function and the act of dreaming, in particular the connection between dreams, memory, and psychological disorders.

The study of dreams became more popular with the discovery of REM sleep, and evidence of increased dreaming during this phase of sleep, when the sleeping brain most closely resembles the awake brain. This type of study includes research into what influences dreaming, the mechanisms at work behind dreaming, and sleep or other disorders that may affect dreaming.

Oneirologists may analyze brain waves visible on an EEG during dreaming, or investigate the effects on dreams of various neurotransmitters and medications.

The Function of Dreams

For centuries, people have attempted to understand and interpret dreams. Ancient Egyptians believed that dreams represented the existence of things that could not be accessed in waking life. Early Christians thought that dreams formed a direct line of communication from God.

The advent of psychiatry brought heightened interest to the subject of dreams, and to interpreting them as a means of accessing the psyche. In his seminal book The Interpretation of Dreams, Sigmund Freud suggested that dreams represent repressed desires and anxieties.

Freud divided dreams into two types: manifest content and latent content. Manifest content is the actual content of the dream including thoughts and images, while latent content describes the subconscious psychological significance of the dream.

Some of the most famous quotes about dreams can be attributed to Freud and fellow psychiatrist Carl Jung. Jung believed that dreams were messages, and recurring dreams a way to confront and resolve persistent fears or other issues.

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Jung also theorized that dreams revealed universal archetypes experienced by all people and cultures. He believed that these archetypes, such as the wise old man, the flood, the trickster, and the shadow, formed the basis of all stories and religions, and could explain dreams, as well.

The Activation Synthesis Theory of Dreams

Proposed by Harvard University psychiatrists John Allan Hobson and Robert McCarley in 1977, the activation synthesis theory of dreams posits that brain activity during REM sleep results in dreams. (8)

Dr. Hobson’s five essential characteristics of dreams include:

  • Intense emotions
  • Illogical content
  • Apparent sensory impressions
  • Uncritical acceptance of dream events
  • Difficulty in being remembered

Hobson and McCarley believe that dreams are caused by the higher brain’s interpretation of the functioning of the more primitive mind. According to their theory, activation in the brain stem during REM sleep causes areas of the limbic system to become active, as well. These include areas involved in the processing of emotion, sensory signals, and memory.

The Activation Synthesis Theory holds that dreams are created from this brain activation during sleep. Dr. Hobson believes that the mind will always try to make meaning of brain activity, and activity that takes place during sleep is no exception.

In recent years, the Activation Synthesis Theory has been updated and renamed the AIM model. AIM stands for activation, input-output gating, and modulation. This three-dimensional model attempts to explain how consciousness shifts through the states of waking, non-REM sleep, and REM sleep.

The AIM model presents the idea that dreaming and brain activity during sleep are essential for the development and operation of consciousness, well as other crucial brain functions such as problem solving. Dreaming is not just what the brain does when it isn’t fully conscious, it is an indispensable part of consciousness itself.


h does a fever cause nightmares and vivid dreams? An elevated body temperature may cause neurons in the brain to transmit signals at a faster rate. This rapid transmission can occur in the visual cortex, as well, contributing to unusually vivid dream imagery and even hallucinations.

Dreams as Therapy

While it may be impossible to entirely understand the nature and purpose of dreams due to their subjective nature, recent understanding of the brain during REM sleep may point to some interesting possibilities.

Centers of memory and emotion become reactivated during REM sleep after decreasing dramatically during non-REM sleep. (9) In addition, noradrenaline, a molecule that triggers anxiety, is absent from the brain during REM sleep. This absence creates a relatively stress-free environment for the processing of emotions or memories that arise during dreams.

SO SleepEdu Dreams Noradrenaline

To help prove the hypothesis that sleep impacts emotion processing, researchers exposed a group of adults to images that induced emotion while their brains were scanned in a MRI machine. The entire group was re-exposed to the images twelve hours later, though only half of the participants had a chance to sleep between the two sessions.

The study subjects who slept reported a decreased emotional response to the same images. This response was supported by the results of their MRI scans, which showed a dramatic reduction in activity in the amygdala, which modulates such primal emotion as fear, anxiety, and aggression. (10) The results of study participants who did not sleep remained similar to the inital results, with similar reactivity in the amygdala.

How do we know dreaming was responsible for the discrepancy in test results? Because only study subjects who slept — and whose dreams showed a decrease in brain activity related to stress — displayed reduced reactivity on the subsequent test.

The Dream Rebound Effect

Dream rebound is the appearance of suppressed thoughts in dreams, particularly those that occur during REM sleep. (11) Suppression of thought and emotion has been shown in multiple studies to impact dreams, particularly when the dreamer is experiencing greater cognitive demands due to the learning of new concepts or memorizing of details or numbers. (12)

Because the usual processes of thought suppression aren’t as firm during REM sleep as they are during wakefulness, suppressed thoughts may be easier for the brain to access and explore. This can lead to the appearance of suppressed thoughts and emotions during sleep. (13Some researchers speculate that negative dreams and nightmares may be at least in part a result of thought suppression.

Several randomized clinical trials have shown that nightmares may be mitigated by reducing the activity of thought suppression. Confronting disturbing thoughts and images while awake instead of attempting to repress them may be helpful to the nightmare sufferer, especially one who experiences recurrent bad dreams. (14)

Imagery rehearsal therapy, or IRT, is a type of cognitive behavior therapy designed for sufferers of negative dreams and PTSD. Instead of suppressing the fears that lead to nightmares, IRT instructs patients to confront them. The object of IRT is to alter the course of dreaming patterns by helping nightmare sufferers reimagine more benign endings for nightmares, and take conscious control of the content of dreams. (15)


A cell in the body that transmits, processes, and receives information through signals from other neurons.

Nightmares, Night Terrors, and Bad Dreams

Nightmares are dreams composed of negative imagery or emotion, and often lead to intense feelings of fear and anxiety. They may be quite vivid, and awaken the dreamer from sleep.

As many as 8% of adults may suffer from more than occasional nightmares. Nightmares may be caused by a number of psychological factors, including stress, anxiety, certain medications, sleep disorders or disrupted sleep, and mental health disturbances. Distress during waking is strongly associated with nightmares during sleep. (16)

Death, threats to safety, and health concerns are common subjects of nightmares. They are more likely than other dreams to involve unhappy endings, along with themes of failure and aggressiveness. (17Nightmares tend to occur later in the night during REM sleep, when dreams are most vivid.

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Nightmares may also be caused by:

  • Medications
  • Substances
  • Alcohol
  • Withdrawal from certain medications or drugs
  • Fever
  • Sleep disorders
  • Circadian rhythm disturbances such as jet leg

Good sleep hygiene may help improve sleep quality and efficiency, and thereby reduce the frequency of nightmares. Good sleep hygiene calls for a dark, quiet, cool bedroom, avoidance of electronics in the hours before bed, limiting alcohol and caffeine use, and maintaining a consistent sleep schedule.

Nightmares Vs. Bad Dreams

What distinguishes nightmares from bad dreams? Often it is the emotion experienced during the dream. Nightmares tend to be distinguished by the elements of physical aggression and fear, while bad dreams often involve emotions common to interpersonal conflict. These emotions include:

  • Sadness
  • Guilt
  • Confusion
  • Disgust

Nightmares are also distinguished from bad dreams by their tendency to wake dreamers from sleep. Studies reveal that bad dreams are more common than nightmares, occurring in 10.8% of people studied vs. 2.9%. Interestingly, issues of competence are more common in ordinary dreams, which contain fewer obvious themes of failure and negativity.

Nightmares Vs. Nightmare Disorder

Also termed dream anxiety disorder, nightmare disorder is characterized by frequent and severe nightmares that interfere with quality of life. The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines the signs and symptoms of nightmare disorder as:

  • Waking up repeatedly, and having clear recall of the nightmare’s contents, with threats to survival, security, or physical integrity
  • Usually occurs in the latter part of sleep or during napping
  • Being alert upon awakening
  • Feeling distressed, including fear and anxiety
  • Impairment of function during work or school or other domains
  • There is no other medical condition or use of medication or substance to explain the disorder

Nightmare disorder is considered a parasomnia, or a disorder that causes unpleasant experiences while falling asleep, during sleep, or while waking up from sleep. It may have similar causes as ordinary nightmares. It may cause a fear of falling asleep, which can result in chronic sleep deprivation and further anxiety.

Treatment for nightmare disorder may include medications, Cognitive Behavioral Therapy, or both. Therapy protocol may involve the following:

  • Image Rehearsal Therapy — the patient writes down their nightmare and changes elements of the dream to make it more positive. This is followed by rehearsing the amended dream so that future  dreams may be altered, as well
  • Self-Exposure Therapy — the patient gradually exposes themselves to situations that usually create fear and anxiety, causing desensitization
  • Lucid Dreaming Therapy – the patient attempts to become aware of their nightmares while asleep so that the dream and its outcome may be changed


What are most dreams about? Studies show that most dreams involve people that the dreamer knows, interacting with the dreamer in some way.

Post-Traumatic Stress Disorder and Nightmares

Post-traumatic stress disorder (PTSD) occurs as a result of trauma, and is common in veterans of war as well as survivors of sexual abuse. The disorder is characterized by persistent and intrusive thoughts, along with flashbacks of the precipitating traumatic event. The disorder may have a genetic connection to the neurotransmitter serotonin, which affects both emotion and sleep.

Nightmares appear in the majority of people diagnosed with PTSD. (18) They may be even more common in PTSD sufferers with panic or anxiety disorder. PTSD may cause increased arousal of the nervous system, resulting in heightened anxiety, insomnia, and frequent awakening. This tendency toward disturbed sleep may exacerbate nightmares, while fatigue may worsen feelings of depression and hopelessness.

Cognitive behavioral therapy specifically developed for PTSD may be quite effective in treating symptoms of the disorder. Repeated exposure to memories may reduce their power, and desensitize the PTSD sufferer to intrusive images, thoughts, and nightmares. Certain medications may help, as well.


Night terrors

Night terrors are a parasomnia that typically affects young children between the ages of two and four. It may cause some or all of the following symptoms:

  • Yelling or screaming in sleep
  • Sitting upright in bed
  • Moving or thrashing in bed
  • Appearing agitated while asleep or after waking
  • Seeming inconsolable during or after a night terror
  • Appearing confused when awakened
  • Sleepwalking or getting out of bed

While a child suffering from a nightmare may be easily awakened and consoled, a child suffering from a night terror is more difficult to wake up. A child who experiences night terrors may also injure themselves during sleep or suffer from bedwetting. Some researchers theorize that night terrors result from normal childhood changes, such as sleeping alone away from parents. (19)

Treatments for night terrors include scheduled awakenings to avoid long periods spent asleep and dreaming, medications, and a machine designed to lightly vibrate when it senses night terrors beginning, awakening the child just enough to interrupt the cycle of frightening dreams.

Last Word from Sleepopolis

While still a mystery, dreams are no longer as puzzling as they once were. We now know that dreams occur in all four stages of sleep, begin in early brain development, and appear to take their subjects in part from suppressed thoughts. Though we no longer believe that dreams are symbols of repressed wishes or communication from the gods, they haven’t lost their power to inspire and make us wonder.

Because dreams are so individual and represent the life and concerns of each person who dreams, we may never fully understand their purpose or meaning. Still, recent advances in sleep science and neurology have given us tantalizing glimpses into the nature of dreams, and the fascinating brain processes that lead to dreaming.


  1. Siclari F, Bernardi G, Cataldi J, Tononi G., Dreaming in NREM Sleep: A High-Density EEG Study of Slow Waves and Spindles, Journal of Neuroscience, Oct. 24, 2018
  2. Suzuki H, Uchiyama M, Tagaya H, Ozaki A, Kuriyama K, Aritake S, Shibui K, Tan X, Kamei Y, Kuga R., Dreaming during non-rapid eye movement sleep in the absence of prior rapid eye movement sleep, Sleep, Dec, 15, 2004
  3. Purves D, Augustine GJ, Fitzpatrick D, et al., editors, Sleep Stages, Neuroscience. 2nd edition, 2001
  4. Hiroyuki Suzuki, PhD., Dreaming During Non-rapid Eye Movement Sleep in the Absence of Prior Rapid Eye Movement Sleep, Sleep, Dec. 2004
  5. Takeuchi T, Ogilvie RD, Murphy TI, Ferrelli AV, EEG activities during elicited sleep onset REM and NREM periods reflect different mechanisms of dream generation. Electroencephalograms. Rapid eye movement, Clinical Neurophysiology, Dec. 2004
  6. E. Garcia-Rill, Reticular Activating System, Encyclopedia of Neuroscience, 2009
  7. Urbano FJ, Kezunovic N, Hyde J, Simon C, Beck P, Garcia-Rill E., Gamma Band Activity in the Reticular Activating System, Frontiers in Neurology, Jan. 31, 2012
  8. J. Allan Hobson, REM sleep and dreaming: towards a theory of protoconsciousness, Nature Reviews Neuroscience, Oct. 1, 2009
  9. Matthew P. Walker, Overnight therapy? The role of sleep in emotional brain processing, Psychological Bulletin, 2009
  10. Ressler KJ, Amygdala Activity, Fear, and Anxiety: Modulation by Stress, Biological Psychiatry, June 15, 2010
  11. Tana Kroner-Borowik, The effects of suppressing intrusive thoughts on dream content, dream distress and psychological parameters, Journal of Sleep Research, Nov. 2, 2012
  12. Richard A. Bryant, Dream rebound of suppressed emotional thoughts: The influence of cognitive load, Consciousness and Cognition, Volume 20, Issue 3, Sept. 2011
  13. Schmidt RE, Gendolla GH., Dreaming of white bears: the return of the suppressed at sleep onset, Consciousness and Cognition, Sept. 2008
  14. Postal self-exposure treatment of recurrent nightmares, British Journal of Psychiatry, Oct. 1994
  15. L Cutchen L, Cheng D, Emmons S, Germain A, Melendrez D, Sandoval D, Prince H., Imagery rehearsal therapy for chronic nightmares in sexual assault survivors with posttraumatic stress disorder: a randomized controlled trial, JAMA, Aug. 1, 2001
  16. Marquis LP, Julien SH, Baril AA, Blanchette-Carrière C, Paquette T, Carr M, Soucy JP, Montplaisir J, Nielsen T., Nightmare Severity Is Inversely Related to Frontal Brain Activity During Waking State Picture Viewing, Journal of Clinical Sleep Medicine, Feb. 2019
  17. Robert G, Zadra A., Thematic and Content Analysis of Idiopathic Nightmares and Bad Dreams, Sleep, Feb. 1, 2014
  18. Detweiler MB, Pagadala B, Candelario J, Boyle JS, Detweiler JG, Lutgens BW, Treatment of Post-Traumatic Stress Disorder Nightmares at a Veterans Affairs Medical Center, Journal of Clinical Medicine, Dec. 2016
  19. Boyden SD, Pott M, Starks PT., An evolutionary perspective on night terrors, Evolution, Medicine, & Public Health, Apr. 14, 2018

Rose MacDowell

Rose is the former Chief Research Officer at Sleepopolis. An incurable night owl, she loves discovering the latest information about sleep and how to get (lots) more of it. She is a published novelist who has written everything from an article about cheese factories to clock-in instructions for assembly line workers in Belgium. One of her favorite parts of her job is connecting with the best sleep experts in the industry and utilizing their wealth of knowledge in the pieces she writes. She enjoys creating engaging articles that make a difference in people’s lives. Her writing has been reviewed by The Boston Globe, Cosmopolitan, and the Associated Press, and received a starred review in Publishers Weekly. When she isn’t musing about sleep, she’s usually at the gym, eating extremely spicy food, or wishing she were snowboarding in her native Colorado. Active though she is, she considers staying in bed until noon on Sundays to be important research.