The Science of Sleep and the Dreaming Brain

The experience of dreaming is nearly universal, yet its origins and purpose remain one of the great unanswered questions in science. 

On average, every individual spends about two hours each night traveling through seemingly real experiences that bubble up from the subconscious. Despite this ubiquity, dream recall is surprisingly low, with most people remembering only about two dreams per week. In rare cases, roughly one in every 250 people has never recalled a single dream.

Dreams largely arise during the Rapid Eye Movement (REM) phase of the sleep cycle, though not entirely. Sleep is a natural process conserved across evolution in nearly all animals with a brain, and the REM cycle repeats about four to six times per night, with each period lasting roughly 90 minutes.

Modern science has evolved from crude mid-1900s methods—like taping participants' eyes open—to using advanced techniques such as MRIs and the electroencephalogram (EEG), which link dreams to observable brain functions. Research suggests that during dream sleep, the brain connects new information learned during the day with already-stored memories, creating a "revised mind-wide web of associations." This process, blending emotion-driven visual imagery and memory consolidation, is theorized to help us make sense of experiences, regulate emotions, and prepare for future situations.5 Specific brain structures are involved in this process. 

The hippocampus, a region critical for memory formation, plays a major role in dreaming; studies show that people with damage to this area still dream, but their narratives lack the richness of detail described by others.

 The body is designed to prevent us from acting out our dreams through temporary paralysis of skeletal muscles (known as REM atonia). Failures in this process lead to certain phenomena, such as sleepwalking (somnambulism) which occurs most frequently in children, but only during the deeper stages of non-REM sleep when the protective muscle paralysis is absent. 

Sleep Paralysis is a well-known phenomenon that occurs when the brain awakens early but the body fails to "unfreeze" in sync, often resulting in terrifying hallucinations.

In contrast, lucid dreaming is a state where the dreamer is aware they are dreaming and can even control their actions. About 20% of people experience this at least once a month, though most people never report the experience. This heightened state is associated with increased activity in the frontal lobes, the brain regions responsible for decision-making and attention management, and may involve the emergence of a collaborative brain network. 

 Nightmares, considered parasomnias (undesirable events experienced during sleep), are unpredictable but have been linked to factors such as stress, anxiety, and trauma.

For most of recorded history, dreams were considered divine in origin, with the earliest record of dream interpretation dating back to Ancient Sumer. The first venture into modern theory was by the ancient Greek philosopher Heracleitus, who proposed that dreams were created within the mind. This idea was formalized with the rise of psychoanalysis in the late 1800s.

Famed Austrian psychologist Sigmund Freud believed dreams were symbolic expressions of desire and that interpreting their content—often using free association—could reveal truths about one's psyche. Separately, psychologist Carl Jung proposed the controversial idea of the 'collective unconscious,' a universal part of the unconscious mind containing innate elements called archetypes (like the Hero or the Shadow) that shape dreams across cultures.

However, the interpretation of dream patterns must also account for a cognitive bias called pareidolia, which is the tendency to find meaning and assume a generative force behind patterns produced by randomness. 

 Finally, while the question of whether animals dream depends on one's definition, studies on sleeping cats, rats, fish, and finches show their brains fire as if performing actions from waking life, and even spiders and insects exhibit REM-like sleep.