From a subjective viewpoint, it can be difficult to believe that dreams have any deep meaning. One of my most memorable dreams involved the actor Tom Selleck, a sea monster and a talking duck. If this was supposed to be a message from my subconscious, its significance was certainly lost on me. Indeed, reasons for dreaming have puzzled philosophers and scientists for many years. We spend nearly twenty-five years of our lives asleep, and dreams are a central part of that experience. It stands to reason that they should serve a purpose. While the debate on the purpose of dreaming is far from over, at last scientists have intriguing insights into how the brain spins its nightly adventure tale.

About 90 minutes after the onset of sleep, several abrupt physiological changes occur. Electroenchepalogram (EEG) readings become desynchronized, showing a low-voltage, fast activity pattern similar to that of the wakefulness. Despite the near-waking levels of brain activity, a person in this stage of sleep is the most difficult to arouse. Dubbed “paradoxical sleep,” this stage is characterized by desynchronized cortical activity, rhythmic hippocampal activity, and loss of core muscle tone. Only the skeletal muscles controlling movements of the eyes, the middle ear ossicles and respiration are not paralyzed. In 1957, William Dement and Nathaniel Kleitman described the association of rapid eye movements (REM) associated with this stage of sleep, and since then it has become known as REM sleep. These rapid eye movements appear to be driven by phasic bursts of electrical activity that can be recorded in animals from a variety of structures in the brain stem, the thalamus and the auditory cortex. These monophasic sharp waves originate in the pons and travel through the lateral geniculate nuclei to the occipital cortex. Called PGO (pontine-geniculate-occipital) spikes, they correlate perfectly with the eye movements recorded in REM sleep.

Dement and Kleitman were curious to see whether the rapid eye movements were associated with dreaming, so they did a controlled study comparing subjects’ responses to awakening from REM sleep versus awakening from non REM sleep. They found that REM sleep was highly correlated with dreaming; people awakened from REM sleep reported dreaming 80% of the time, whereas people awakened from non-REM sleep reported dreams only 7% of the time.

Facts and Myths about Dreaming
As anyone who has had the sound of their alarm clock turn into a train whistle or fire alarm can attest, external stimuli can sometimes be incorporated into a dream’s content. Dement empirically tested this phenomenon in 1958 by spraying water on sleeping subjects after they had been in REM sleep for a few minutes. A few seconds after the spray, the subjects were awakened and asked to recount their dream. In 14 of 33 cases, the water was incorporated into the dream. One subject reported the following:

“I was walking behind the leading lady when suddenly she collapsed and water was dripping on her. I ran over to her and water was dripping on my back and head. The roof was leaking. I looked up and there was a hole in the roof. I dragged her over to the side of the stage and began pulling the curtains. Then I woke up.”

But dreams do not capture the full sensory experience of wakefulness. For example, we rarely experience pain in our dreams. This is probably because pain is a signal that something is wrong, and we need to be able to react immediately in such a situation. A sleeper who is bitten by a snake better wake up instantly rather than weave the pain of the bite into the dream. Some researchers have proposed that the fragile memories of dreams also serve a protective function. Without rehearsal, most dreams disappear from memory within 8 minutes, possibly because they are not well-integrated with other memory traces. The evaporation of dream memories reduces the probability of confusing dreams with reality.

Dement also tested the common idea that dreams do not run on “real time.” With all of the strange events that occur in dreams, it can often feel like time is disjointed — events that should take days may seem to pass in seconds. Or do they? Dement examined this question by counting the number of words in the dream reports and comparing them with the amount of time the dreamer had been in REM sleep. The length of the dream narratives showed a high positive correlation with the duration of REM sleep, so Dement probed further. He awakened subjects either 5 or 15 minutes after they entered REM sleep and asked them to pick which time they thought reflected the amount of time they had been dreaming. The subjects picked correctly in 83% of the trials, indicating that dream time is not as compressed as some have believed.

Why Do We Dream?
Philosophers and writers as far back as Aristotle have speculated on the reason for dreams, but the first theory to really catch fire was the one proposed by Sigmund Freud at the turn of the century. Freud believed that people dream to relieve sexual frustration created by repressed desire, allowing them to act on forbidden impulses. However, he felt that because the rules of polite society reject such impulses, people had to disguise their true feelings using symbolic imagery. So instead of dreaming about sexual intercourse, a person might dream about a train entering a tunnel. Freud’s theory has been widely disseminated and popularized through literature, television and movies, but most contemporary psychologists feel that it is riddled with problems. First, almost all mammals dream, and it is hard to believe that our pets feel the societal pressure to conform to sexual mores. This argument applies to infants as well, since fetuses and newborns spend twice as much time dreaming than adults do. Furthermore, not all dreams involve sex or sexual imagery. Even Freud admitted, “Sometimes a cigar is just a cigar.”

In the late 1970s, Hobson and McCarley proposed the activation-synthesis hypothesis of dreaming, which maintains that a dream is simply a reflection of the brain’s aroused state during REM sleep. During this period, the cerebral cortex is active but is largely shut off from sensory input. Internal stimuli such as memories become more prominent because they do not have to compete with information pouring in through the senses. According to Hobson and McCarley, the memories most likely to come to mind are the most recent ones, which would explain why we often dream of the previous day’s events. The cortex performs its usual job and attempts to integrate the messages into a coherent narrative, but the fractured images combine in bizarre ways that make sense only within the context of the dream. However, dreams are not objective playbacks of our memories. They are stories colored by thoughts, hopes and wishes that have deeply personal meaning. This would explain why we often dream about people, places and events that have emotional significance for us.

Another theory about dreaming comes from the field of memory research. This theory holds that the reiteration of memories in our dreams serves to solidify the memory’s storage. Support for this idea comes from studies where subjects learned a set of words before undergoing a period of REM sleep deprivation. The group that did not get any REM sleep recalled fewer words than the group that was allowed to experience REM sleep. These findings prompted neuroscientists to examine the brain for patterns of neuronal activity that occurred both during waking and sleeping. This is a difficult task even in a rat brain, since there are billions of possible synaptic connections. Where to look first? In Bruce McNaughton’s lab, they concentrated on spatial learning and the hippocampus. They found that neuronal circuits in the hippocampus that were active while the rat was learning a spatial task (such as a maze) were reactivated when the animal went to sleep. These results are exciting, but so far there is no hard evidence that memory reactivation during sleep has functional significance. While the REM sleep deprivation experiments would suggest that sleep and memory are connected, such findings are always difficult to interpret because lack of sleep usually makes people irritable and less focused.

The elusive and personal nature of dreams makes them challenging to study, but researchers are making inroads toward unlocking their meaning. Until then they remain tantalizing glimpses into the world of the sleeping brain, a mystifying and mesmerizing nightly movie theater for one.

References:
Clincial Physiology of Sleep, Ralph Lydic and Julien F. Biebuyck, Eds. Baltimore: Waverly Press, 1988.

Kandel, E. R., Schwartz, J. H, and Jessel, T. M. Principles of Neuroscience, Third Ed., Appleton & Lange, 1991