At present, we still do not have a precise definition of dreams; people in different eras and regions have interpreted dreams in various ways, and there are even vast differences in the understanding of dreams across different religious and cultural contexts.

In Aboriginal Australian mythology, a dream is a ‘timeless time’ created by an individual or a group, within which primordial or eternal creations are brought into being.

Around 3100 BC, the Sumerians began recording some of their dreams on clay tablets. They believed that whilst we sleep, our soul—or a part of it—leaves the body, and what we see in our dreams are the places and people the soul actually encounters.

The Babylonians and Assyrians categorised dreams as ‘good dreams’ originating from the gods and ‘bad dreams’ originating from spirits; they also regarded dreams as a source of omens and prophecies.

The ancient Egyptians would record their dreams on papyrus; these vivid dreams were regarded as blessings from the gods. They believed dreams to be divine oracles capable of conveying messages from the gods; consequently, they employed special ‘dream beds’ in temples to perform ‘dream incubation’ rituals, through which they sought divine guidance, comfort and instruction.

In ancient China, there was a belief that during sleep, the human soul split into two parts: one part wandered outside the body, whilst the other remained within. However, this interpretation has been the subject of much controversy since ancient times.

The Indian Upanishads emphasised two aspects of dreams: firstly, that dreams are an expression of inner desires; and secondly, that before waking, the soul leaves the body and receives guidance.

The ancient Greeks carried on the ancient Egyptians’ beliefs regarding good and bad dreams, as well as the ritual of ‘incubation’. They believed that the gods would personally visit the dreamer’s dreams, entering and departing via the same path. Morpheus, the god of dreams, was regarded in Greek culture as a being capable of bestowing warnings and prophecies upon those sleeping in sanctuaries and temples.

The ancient Greek writer Antiphon, in the 5th century BC, wrote what is currently recognised as the first known Greek book on dreams.

During the same period, many philosophers offered their own interpretations of dreams.

Hippocrates simply explained dreams as the soul absorbing images during the day and producing them at night.

Aristotle, on the other hand, believed that dreams were related to physiological activity and utilised them to analyse and predict illness.

Cicero believed that all dreams originated from the dreamer’s daytime thoughts and conversations; in The Dream of Scipio, he described the imagery of a lengthy dream.

Some Native American tribes and Mexicans regard dreams as a means of communicating with their ancestors. In some Native American tribes, vision quests are practised as rites of passage; participants fast and pray continuously until they receive the desired guidance in a dream, after which they return to inform the other members of the tribe.

The ancient Hebrews closely linked dreams to their faith, regarding them as the voice of God; they believed that good dreams came from God whilst bad dreams came from evil spirits. Like many ancient civilisations, they practised dream incubation to receive divine revelation. For example, the Hebrew prophet Samuel fell asleep by the Ark of the Covenant in the Temple of the Lord and heard God’s call.

In the Middle Ages, people regarded dreams as evil, and the images in dreams were considered to be the temptations of the devil. Some believed that the devil would instil corrupt and harmful thoughts into the minds of those asleep.

In the late 19th century, the Austrian psychologist and psychotherapist Sigmund Freud put forward a theory positing that dreams are expressions of unconscious psychological conflicts and desires, and that through dreams, individuals can release repressed desires. He also described dreams as a highway to the unconscious, believing that the content of dreams is shaped by unconscious desires.

Significant unconscious desires are typically linked to childhood memories and experiences. Furthermore, he categorised dreams into manifest and latent dreams, arguing that latent dreams are associated with desires or fantasies deep within the unconscious, whilst manifest dreams are superficial and lack deeper meaning.

Published in 1899, The Interpretation of Dreams is one of Freud’s seminal works; in this book, he explored theories concerning the psychology of dreams and psychoanalysis. Freud emphasised that dreams frequently contain expressions of sexual desires, which may be repressed by the individual whilst awake. This perspective has had a profound influence on subsequent thinking regarding dreams and psychoanalysis.

Carl Jung, however, held views that differed somewhat from those of Freud.

He described dreams as a means of conveying messages to the dreamer, aiding in self-improvement; dreams reveal the dreamer’s emotional or spiritual issues and fears. Recurring dreams deserve our attention, as they indicate that the dreamer may have overlooked the themes that recur in their dreams within their daily life.

Jung also believed that dreams do not exist in isolation but form part of a network of psychological factors; every person and inanimate object in a dream is an extension of the dreamer, thereby emphasising the subjective nature of dreams.

As our understanding and research into sleep have deepened, scientists have discovered that during sleep we pass through multiple sleep stages, including rapid eye movement (REM) and non-rapid eye movement (NREM) stages. A complete sleep cycle lasts 90 minutes.

Some physiological theories suggest that dreams may be linked to the REM sleep stage, during which our brains remain in a state of active wakefulness—a phenomenon that has been confirmed through electroencephalography (EEG). Dreams may be a way for our brains to integrate and process the information and experiences accumulated during the day, enabling us to better regulate the emotions that arise.

The total duration of dreaming over a person’s lifetime is typically around six years, which equates to two hours of dreaming per night.

Apart from humans, many animals may also possess the ability to experience REM sleep and produce dreams. Scientific research shows that all mammals experience REM sleep, although the duration of REM sleep varies between species: dolphins have relatively short REM periods, humans fall in the middle, whilst armadillos and North American opossums have some of the longest.

Furthermore, evidence of dreaming has been found in studies of monkeys, dogs, cats, rats, elephants and shrews, as well as in birds and reptiles; scientists have even observed a sleep state similar to REM for the first time in the arachnid mantis.

In 1966, Calvin S. Hall and Van De Castle published The Content Analysis of Dreams. By analysing over 50,000 dream reports collected at Case Western Reserve University, they discovered similarities in the content of dreams reported by participants from different regions.

Dreams are highly visual; images of different places and objects often blend together, and the imagery presented in dreams—which is not tied to specific locations or objects—is exaggerated and bizarre. The dreams of people who have been blind from birth contain no visual imagery but are instead associated with other senses, such as hearing, touch, smell and taste.

Hall’s research found that negative emotions are more common in dreams than positive ones. Anxiety is the most frequently experienced emotion in dreams, whilst other emotions include feelings of abandonment, anger, fear and joy.

Lucid dreaming, also known as clear dreaming, refers to a state in which one remains conscious within a dream and is aware that one is dreaming.

Some people suddenly realise they are dreaming whilst asleep; once they recognise this, they are able to consciously control the events of the dream—this is the experience of lucid dreaming. During lucid dreaming, the individual can freely direct their own actions, manipulate the dream’s events at will, and even exert complete control over other characters within the dream.

It should be noted that lucid dreaming does not necessarily require the ability to manipulate objects within the dream; simply being aware that one is dreaming is sufficient for it to be classified as lucid dreaming.

In 1975, psychologist Keith Hearne and his assistant Alan Worsley conducted an experiment with a lucid dreamer. They agreed with the subject that, whilst asleep and during a lucid dream, the subject would move their eyes from side to side; this would allow them to verify that the subject could indeed control their body within the dream, and the subject successfully carried out this agreed signal. This experiment confirmed the existence of lucid dreaming.