If you look in the dictionary, you’ll find the definition of dreams as follows: a series of images, ideas, emotions and sensations occurring involuntarily in the mind during certain stages of sleep. The definition of dreams today also encompasses daydreams and states of trance and abstraction, Psychologists have defined dreams as mental activity, usually involving a series of imagined events, that occurs in certain phases of sleep. In to the past dreams have been defined in all sorts of different ways in different cultures. It seems that all human cultures have attached some kind of significance to dreams; as prophecies, communication with the spirit world, messages from the self or even messages from demons or gods.

Many indigenous cultures around the world define dreams as gateways through which the human dreamer can access other worlds and communicate with supernatural entities. The ancient Sumerians shared this view: archaeologists have excavated some inscribed clay tablets from Mesopotamia that seem to be the earliest dream journals. The Sumerians believed that, during dreams, the soul left the body on a kind of astral journey, during which time the dreamer physically visited the places and people that he or she dreamed about. In Sumeria and other civilizations of the ancient Near East, dreams were supposed to be the messages from gods or demons and were often prophetic in nature. Egyptians also saw dreams as messages in which the gods revealed themselves, either to give a warning or to demand a devotional act. Some Egyptian temples had “dream beds” where pilgrims could sleep in hopes of receiving a dream of advice, healing or prophecy.

The idea of dreams defined as prophecy is also common in Hebrew and Christian texts, which feature stories in which a prophet or religious leader might receive a dream in answer to a prayer or when faced with a dilemma. In early monotheistic religion, dreams are usually characterized as the word of God, delivering a prophecy or solution to a dreamer who seeks Him out. However, in the Middle Ages a harsher definition of dreams held sway: religious figures condemned dreams as temptations sent by the devil to impressionable dreamers. However, some religious figures like St. Augustine disagreed, claiming that his dreams had helped him along his religious path.

In animistic or pantheistic religions, many different gods, spirits or ancestors may speak to the dreamer to help him or her solve a problem or to predict the future. Native American and Mesoamerican civilizations believed that dreams provided ways to communicate with ancestors;  indeed, the Chontal Indians of Oaxaca still use the herb Calea zacatechichi to access their ancestral dream realms.

The Greek physician Hippocrates may have created one of the earliest theories of dreams; he posited that in the daytime the soul receives images and at night it creates them – a rough precursor of the current psychological model of dreaming, in which it is said that our subconscious takes experiences from our waking lives and recycles them into dreams as we sleep. Greek philosopher Aristotle also hypothesized that dreams were manifestations of physiological activity and that their interpretations could be used to diagnose and treat illnesses.

In 19th century Europe and North America, dreams and dream interpretation had fallen out of favor for some time when Sigmund Freud published his landmark book The Interpretation of Dreams. Before Freud, people often didn’t attach much significance to their dreams; they thought they were simply a byproduct of anxiety, family issues, or perhaps just a result of something they ate. You may be familiar with the scene in A Christmas Carol where the protagonist Scrooge is confronted by the ghost of his dead business associate Marley in a dream: he blames Marley’s appearance on nothing more than “a spot of mustard, a bit of ham” in the sandwich he had before bed! Sigmund Freud created a revolutionary definition of dreams as products of our subconscious, and argued that the interpretation of dreams could help in understanding the subconscious self.

Modern brain imaging and brainwave measuring techniques have let researchers observe what happens in the brain when we sleep and dream. Eugene Aserinsky first observed Rapid Eye Movement in 1952 when he was working in his PhD supervisor’s operating room; he later conducted experiments, such as waking people in the middle of REM, to verify that they were dreaming. However, even now there is no universal biological definition of dreams. Oneirologists (people who study dreams) can agree that most dreams happen during REM sleep, when the sleeping brain is most near to being awake and aware.

There have been several competing theories about what mechanisms in the brain generate dreams. Many scientists have supposed that dreams may be the result of neural processes going on in the brain: one theory is that dreams represent the forebrain’s attempt to interpret nerve impulses from the brain stem or the parietal lobe in the absence of sensory information. Dreams may also be a way for the brain to convert memories from short term to long term, to consolidate long term memories, or to link distant but related memories. An opposite theory to memory-consolidation is that dreams create a space for the subconscious to process “junk” from everyday life, such as underdeveloped thoughts and sensory impressions, instead of storing it in long term memory. In other words, we dream to forget. Not a far fetched idea, considering how much effort it can take to remember dreams!

The definition of dreams has changed as much as people’s thoughts, concerns and ways of seeing the world have changed. Whatever dreams are, they provide us with a way of inquiring within that still has a lot to teach us about ourselves and our relationship to the reality around us.