For centuries people have pondered the meaning of dreams. Early civilizations thought of dreams as a medium between our earthly world and that of the gods. In fact, the Greeks and Romans were convinced that dreams had certain prophetic powers. Over the years, numerous theories have been put forth in an attempt to illuminate the mystery behind human dreams. Only recently has strong, tangible evidence become less elusive.

The long-standing mystery of dreams

Since ancient times, humans have been trying to understand the purpose of dreams.

Psychological Theories of Dreaming

Freudian and Jungian Theories

While there has always been great interest in the interpretation of human dreams, it was not until the end of the nineteenth century that Sigmund Freud and Carl Jung put forth some of the most widely-known modern theories of dreaming. Freud's theory centered around the notion of repressed longing or wish fulfillment—the idea that dreaming allows us to sort through unresolved, repressed wishes. Freud's theory described dreams as having both latent and manifest content. Latent content consists of deep unconscious wishes or fantasies, while manifest content is superficial and meaningless. Manifest content often masks or obscures latent content. It was in his book The Interpretation of Dreams (published in 1900) that Freud first argued that the motivation of all dream content is wish-fulfillment, and that the instigation of a dream is often to be found in the events of the day preceding the dream, which he called the "day residue." Later, Freud revised his theory to suggest that dreams may also represent the repetition compulsion, which is a psychological phenomenon in which a person repeats a traumatic event or its circumstances over and over again. This can take the form of reenacting the event—which can include "reliving" the scenario in the form of dreams—or putting oneself in situations where the event is likely to happen again.

Carl Jung (who studied under Freud) also believed that dreams had psychological importance, but proposed different theories about their meaning. Jung expanded on Freud's idea that dream content relates to the dreamer's unconscious desires. He thought of dreams as messages to the dreamer, containing revelations that could uncover and possibly resolve emotional or religious problems and fears.

Threat-Simulation Theory

Threat-simulation theory suggests that dreaming should be seen as an ancient biological defense mechanism. Dreams are thought to provide an evolutionary advantage because of their capacity to repeatedly simulate potential threatening events. This process enhances the neurocognitive mechanisms required for efficient threat perception and avoidance.

During much of human evolution, physical and interpersonal threats were serious enough to reward reproductive advantage to those who survived them. Therefore, dreaming evolved to replicate these threats and allow people to regularly practice dealing with them. This theory suggests that dreams serve the purpose of allowing for the rehearsal of threatening scenarios in order to better prepare an individual for real-life threats.

Expectation-Fulfillment Theory

This theory posits that dreaming serves to discharge emotional arousals (however minor) that haven't been expressed during the day. This practice frees up space in the brain to deal with the emotional arousals of the next day and allows instinctive urges to stay intact. In effect, the expectation is fulfilled (the action is "completed") in a metaphorical form so that a false memory is not created. This theory explains why dreams are usually forgotten immediately afterwards.

Neurobiological Theories

Activation-Synthesis Theory

One prominent neurobiological theory of dreaming is the activation-synthesis theory, which states that dreams don't actually mean anything. They are merely electrical brain impulses that pull random thoughts and imagery from our memories. The theory posits that humans construct dream stories after they wake up, in a natural attempt to make sense of the nonsensical. However, given the vast documentation of the realistic aspects of human dreaming, as well as indirect experimental evidence that other mammals such as cats also dream, evolutionary psychologists have theorized that dreaming does indeed serve a purpose.

Continual-Activation Theory

The continual-activation theory proposes that dreaming is a result of brain activation and synthesis. Dreaming and REM sleep are simultaneously controlled by different brain mechanisms. The hypothesis states that the function of sleep is to process, encode, and transfer data from short-term memory to long-term memory through a process called consolidation. However, there is not much evidence to back this up. NREM sleep processes the conscious-related memory (declarative memory), and REM sleep processes the unconscious related memory (procedural memory).

The underlying assumption of continual-activation theory is that, during REM sleep, the unconscious part of the brain is busy processing procedural memory. Meanwhile, the level of activation in the conscious part of the brain descends to a very low level as the inputs from the senses are basically disconnected. This triggers the "continual-activation" mechanism to generate a data stream from the memory stores to flow through to the conscious part of the brain.