“We wake from one dream into another dream.” – Ralph Waldo Emerson
From ancient times (when dreams were considered to hold prophetic powers) to the neurological phenomena studied today, dreams remain one of psychology’s and biology’s most enduring mysteries. Dreams have probably been of interest as long as people have been conscious, which is to say, as long as they have distinguished between dreams and ordinary waking life. Dreaming may be defined as a mental state - an altered state of consciousness, which occurs during sleep.
Dreams usually involve fictive events that are organized in a story-like manner, characterized by a range of internally generated sensory, perceptual, and emotional experiences.1 Throughout the past few decades, several biological and psychological theories have been proposed about the purpose of dreaming and its regulatory effect on the psyche and physiology.2, 3
Earlier theories by psychoanalytic scholars such as Sigmund Freud and Carl Jung suggested that dreams constitute a meaningful reflection of unconscious processes, while others have argued that dreams are not inherently meaningful. This article examines some of the bioregulatory mechanisms – physical and non-physical – associated with dreaming.
Many ancient cultures have attributed symbolic meaning and even prophetic significance to dreams.4, 5, 6, 7, 8, 9 Attitudes to dream evaluation vary depending on the culture from which they arise. Dreams are considered important, real, and public in some cultures, but nonsignificant and personal in others.
While there has always been a great interest in the psychology of human dreams, it was not until the end of the 19th century that Sigmund Freud and Carl Jung put forth some of the most widely-known modern theories of dreaming. Dreams, Freud famously stated, are "the royal road to the unconscious."10 Freud held that one purpose of dreams was to allow the psyche to take un-acted upon impulses and desires and fulfill them away from the conscious mind. Essentially, Freud considered dreaming an expression of repressed conflicts or desires. He believed that the basic function of dreaming was to allow the discharge of repressed instinctual impulses in such a way as to preserve sleep, and that the instigating force causing dreams to occur was always an instinctual, unconscious wish. Freud considered these unconscious wishes to be predominantly sexual in nature. For Freud, because dreams contained unacceptable and unpleasant wishes, this explained why dreams are so regularly and so easily forgotten.11
Carl Jung (1875 -1961), a Swiss psychoanalyst, was a friend and follower of Freud, but soon developed his own ideas about how dreams are formed. The main difference between the two great thinkers was that Jung, unlike Freud, believed the unconscious mind is not animalistic in nature, and that our individual primordial behaviors are not driven by instinct, sex or violence. Jung also believed that dreams had psychological importance, but proposed different theories about their meaning. Unlike Freud, who often suggested that specific symbols represented specific unconscious thoughts, Jung believed that dreams can be highly personal and that interpreting these dreams involved knowing a great deal about the individual dreamer.
Jung believed that dreams had their own language. Jung saw dreams as the psyche’s attempt to communicate important things to the individual, and he valued them highly, as a way of knowing what was really going on. The things we see in our dreams are not signs that represent one specific idea, but rather fluid images to which we ascribe meaning based on our individual experiences. For Jung, dreams may reveal truths, philosophical revelations, illusions, fantasies, memories, plans, irrational experiences or even prophetic visions. They are a natural expression of our imagination and use the most straightforward language at our disposal: mythic narratives. Because Jung rejected Freud’s theory of dream interpretation (that dreams are designed to be secretive), he also did not believe dream formation is a product of discharging our tabooed sexual impulses. Jung took a more rigorous approach, explaining dreams as a sort of “shaped energy," inchoate emotions or thoughts released by the deep subconscious and entrained into narratives by higher regions of the brain. Jung did not believe that dreams need to be interpreted for them to perform their function. Instead, he suggested that dreams are doing the work of integrating our conscious and unconscious lives, and he called this the process of individuation.12, 13, 14
Biology of Sleep and Dreaming
In 1913, French Scientist Henri Pieron authored a book entitled “Le problème physiologique du sommeil," which was the first text to examine sleep from a physiological perspective. This work is usually regarded as the beginning of the modern approach to sleep research. Dr. Nathaniel Kleitman, known as the “Father of American sleep research," began work in Chicago in the 1920s questioning the regulation of sleep and wakefulness, and of circadian rhythms. Dr. Kleitman’s early work included studies of sleep characteristics in different populations and the effect of sleep deprivation. In 1953, Dr. Kleitman, together with Eugene Aserinsky, discovered that dreaming was associated with REM sleep. REM refers to "rapid eye movement," the darting of the eyes under closed lids. They found that sleepers could recall dreams most frequently if they were awakened when their eyes appeared to be moving rapidly beneath their eyelids. This discovery in turn gave researchers a tool with which to monitor dreams.15
As a process, sleep is cyclical. It is divided into REM sleep and 3 stages of non-REM; each has a distinct brainwave frequency and associated physiology. The discovery of REM sleep and its close association with dreaming, and the subsequent elaboration of the non-REM (NREM) / REM sleep cycle has ushered in a new era in the study of dreams. Although dreams have been reported from all sleep stages in laboratory experiments, most dreaming occurs during REM sleep. Generally dreams that occur during NREM sleep are less formed and structured and recall of the content of those dreams is much more limited.16
In evolutionary terms, REM sleep seems to be relatively recent, and has been identified in humans, other warm-blooded animals, and birds. Earlier studies have suggested it appears early in life, in the third trimester in humans, and research has produced evidence the brain of the fetus may in a sense be “seeing” images long before its eyes are opened. Thus, the REM state appears to help the brain build neural connections, especially in the visual areas.
REM sleep is characterized as global high-frequency and low amplitude electroencephalogram (EEG) activity (similar to the waking state), as well as increased heart rate, respiratory activity, and muscle atonia (i.e. temporary muscular paralysis). In this state we dream the most and our brain activity resembles that of waking life. Yet, at the same time, our muscles go slack and we lie paralyzed as if our brain is protecting our bodies from acting out the stories we dream.
It is important to note, however, that REM sleep and dreaming can be dissociated: lesions in the forebrain can leave REM sleep intact while dreaming ceases, whereas brain stem lesions can prevent REM sleep from occurring while individuals continue to report dreams after awakening.17
Tactile percepts, odors, tastes, as well as pleasure and pain are not as commonly reported following REM sleep awakenings.18, 19 Oftentimes the sensational and perceptual experiences of the dream world are unlike those which occur in the world of wakefulness. Alterations from waking life experiences include sensory distortions, misidentifications of characters and places, changes in spatio-temporal integration (e.g. the integration of time and location of an event), misbinding of objects’ features, dissociation, and transpositions (e.g. frequent and abrupt changes in the dream narrative).20
Reports following REM sleep awakenings consistently contain more emotional content than those following NREM sleep.21 Dreamers tend to report elevated levels of joy, surprise, anger, fear, and anxiety, whereas sadness, guilt, and depressed affect tend to be less common.22, 23, 24 A proposed explanation for this emotional finding could be related to a less critical self-reflection during dreams.25
Because REM dream reports frequently contain fear and anxiety-related elements, it has been suggested that the realistic representation of fear in dreams and nightmares serves as a threat simulation in a harmless environment in order to prepare individuals for dangerous situations in real life.26, 27, 28 It has also been shown that several periods of dreaming during one night may be related to the same emotional conflict.29 This can also take the form of a recurring dream.
Non-REM Sleep (NREM)
NREM sleep is now commonly divided into three different stages designated as N1, N2, and N3. The third stage (N3 sleep), also known as deep sleep or slow-wave sleep, was referred to as NREM sleep stages III and IV in earlier terminology30 and is in several ways physiologically distinct from REM sleep. NREM sleep is characterized by a global low frequency and high amplitude EEG signal, slow and regular breathing and heart rate, as well as low blood pressure.
During the sleep-onset phase (N1), individuals frequently experience hypnagogic hallucinations while being unaware that they have already fallen asleep.31 These experiences share some similarities with dreams during REM sleep in terms of dream bizarreness, but are typically characterized by emotional flatness.32, 33 Sleep stage N1 reports frequently contain accounts of dreaming (80–90% of the time), but these reports tend to be shorter than those following periods of REM sleep. Reports after awakenings from NREM sleep N3 contained accounts of dreaming 50–70% of the time.34 Only a few reports contained elements of dreaming after awakenings from N3 sleep early during the night, when large slow waves are most prevalent in the EEG signal.35
Duration and Sequence of Sleep Stages
Sleep progresses in a series of four or five more or less regular sleep cycles of non-REM and REM sleep throughout the night, sometimes referred to as ultradian rhythms (“ultradian” meaning within a day). The first sleep cycle is typically around 90 minutes in length, with the succeeding cycles averaging around 100-120 minutes, (although some individuals may have longer or shorter average cycles), and they are usually shorter in children. Each cycle follows the stages of non-REM sleep (stage N1 – stage N2 – stage N3) and then, after a period in deep stage 3 slow-wave sleep, back through the stages (stage N3 – stage N2 – stage N1). Thus, as the night progresses, the time spent in deep stage 3 sleep decreases and the time spent in REM sleep increases, so that there is a greater proportion of stage 3 sleep earlier in the night, and a greater proportion of REM sleep later in the night, particularly during the final two sleep cycles.
A typical hypnogram showing sleep stages and cycles in adult sleep
(Images by Luke Mastin)
Each sleep cycle is made up of several different stages of non-REM and REM sleep, the overall proportions of which are shown in this pie chart.
For most of the 20th century, scientific dreaming theories were mainly psychological. Over the last few decades, numerous other theories have been put forth to illuminate the mystery behind human dreams. Modern psychologists and neurologists, armed with imaging equipment including PET scans, MRIs and EEGs, have taken things to a deeper and more technical level, speculating that dreaming is the brain’s way of dumping excess data, consolidating important information, and keeping us alert to danger and more.
Since the discovery of rapid eye movement sleep, the neural underpinnings of dreaming have become increasingly better understood. In 1977, Drs. Allan Hobson and Robert McCarley of Harvard University presented a more mechanistic, neurophysiological model of the dream process they called “the activation-synthesis model of dream production.” They proposed the primary motivating force for dreaming is not psychological, but rather physiological, since the time of occurrence and duration of dreaming sleep are quite constant, suggesting a preprogrammed, neurally-determined genesis. (Many counter-argue by saying that this predicted dream occurrence does not account for the content of the dream that likely is psychologically important.)
Hobson and McCarley suggested that the occurrence of dreaming sleep is physiologically determined by a “dream state generator” located in the brain stem. This brain stem system periodically triggers the dream-state with such predictable regularity that Hobson and McCarley were able to mathematically model the process to a high degree of accuracy. During the REM periods produced when the dream-state generator is switched “on,” sensory input and motor output are blocked, and the forebrain (i.e., the cerebral cortex, the most advanced structure in the human brain) is activated and bombarded with partially random impulses generating sensory information within the system. The activated forebrain then synthesizes the dream out of the internally generated information, trying its best to make sense out of the nonsense it is being presented with.36
More recent theories suggest that dreams fulfill an adaptive function related to emotion regulation, hormone regulation, learning, and memory consolidation.37, 38 Other theories propose that dreaming may play an important role in reactivating and further consolidating novel and individually relevant experiences that occurred during waking hours.39, 40 Dreams might also constitute a biological defense mechanism, which has evolved as a capacity to repeatedly simulate threatening situations.41
Brain Imaging Sleep Research
The development and application of brain imaging techniques is revealing another very significant new body of findings concerning the functioning of the brain during REM sleep. Although several research groups have utilized different imaging techniques and subject procedures, striking consistencies have emerged. The groups of Maquet, Braun, and Nofzinger all report a very specific, selective pattern of activation of forebrain structures, suggesting that the brain is organized to carry out particular functions in a concerted fashion.42, 43, 44 Structures in the brain stem, thalamus, and basal forebrain that mediate arousal are also activated. A major role for emotion and drive is suggested by high levels of activation of parts of the hypothalamus and the limbic and paralimbic systems. It is also demonstrated that the amygdala may be particularly connected to the role of anxiety in dreaming.
Maquet and Braun also found a widespread deactivation of the dorsolateral prefrontal cortex, which correlates well with the diminished executive functioning in dreams. Maquet et al, emphasizing the role of the amygdala, suggest that REM sleep is involved in processing ‘‘emotionally significant memories.’’45
Nofzinger and colleagues interpret the pattern of activation as supporting the view that one function of REM sleep is the integration of neocortical activity with hypothalamic-basal forebrain regulatory and motivational reward mechanisms. They see their findings as consistent with a role for REM sleep in memory consolidation, and specifically dream content, associated with internally generated, or instinctual behaviors that subserve adaptive mechanisms.46
A special form of dreaming is the lucid dream, in which the dreamer is aware that he or she is dreaming. Lucid dreamers can remember the events of waking life while dreaming, although they have no awareness of the real external world in which they are sleeping. Also, they appear to be able to control the plot of the dream while remaining asleep. Lucid dreaming is relatively rare, but most people have had at least one lucid dream, and it has been estimated that about 20% of the population dream lucidly at least once per month.47
According to the most frequently used sleep scoring criteria, lucid dreaming is considered a part of REM sleep and usually transpires during late night REM sleep periods.48, 49 However, some research suggests that lucid dreaming may also occur during periods of NREM sleep.50, 51
Lucid dreaming has a special status compared with non-lucid REM and NREM d