What counts as a dream
In research, a dream is generally defined as an experience that occurs during sleep, often involving imagery, a storyline and emotion, which the sleeper believes to be real while it is happening. Because dreams are private, scientists depend mainly on reports given after waking, usually in questionnaires, diaries or laboratory awakenings. This method has well-known limits: memories of dreams fade quickly, and people may fill in gaps or edit the story when describing it. All findings about dream content should be read with that in mind. The sleep stages in which dreams occur are covered in the article on REM sleep dreams, so this piece concentrates on content and theories.
What dreams tend to contain
Large collections of dream reports, gathered from many people over decades, show some consistent patterns.
- Mostly ordinary settings. Many dreams involve everyday places, familiar people and common activities, instead of fantastic scenarios.
- Visual and auditory experience. Vision is typically prominent, hearing is common, and touch, taste and smell appear less often, though people differ.
- Emotion. Negative emotions such as anxiety, fear and anger are often reported at least as frequently as positive ones, depending on the study.
- Social interaction. People, and interactions with them, appear in a large share of reports.
- Continuity with waking life. Recent events and ongoing concerns frequently influence dreams. Researchers sometimes call this the continuity hypothesis. Often the link is loose, and the connection to waking life can be hard to recognize.
- Bizarre elements. Sudden changes of scene, impossible events or odd combinations of people occur in many dreams but are not constant, and dreamers usually accept them without surprise.
Some themes, such as being chased, falling or being unprepared, are commonly reported in many societies. However, reported frequencies differ across studies and populations, and surveys can be biased toward memorable dreams.
Special kinds of dreams
Nightmares are distressing dreams that often wake the sleeper. Occasional nightmares are common. If they are frequent, or linked to stress or trauma, talking to a health professional is sensible.
Lucid dreams are dreams in which the sleeper realizes they are dreaming and can sometimes influence the action. Laboratory studies, including those in which lucid dreamers signaled with eye movements, have shown that these experiences are real and occur during REM sleep. Some people can learn techniques to increase them, though results are mixed.
Recurring dreams repeat similar themes, often related to ongoing worries, though their causes are not well established.
What is happening in the brain
During REM sleep, brainstem circuits help generate the characteristic pattern of activation, and imaging studies suggest increased activity in areas involved in emotion and sensory imagery, with relatively reduced activity in parts of the prefrontal cortex tied to planning and self-monitoring. The neurotransmitter balance differs from waking: levels of some modulating chemicals, such as noradrenaline, fall substantially during REM. These findings help explain why dreams are vivid and emotional yet illogical, but they are descriptive, and the link between particular activity patterns and particular dream content is only starting to be examined. Some studies using brain scans have been able to predict broad categories of dream content from activity, but these are early and limited results.
Leading theories
The question of why we dream has several competing answers, and none has been confirmed.
- Memory consolidation. Dreams may reflect the brain reprocessing experiences. Sleep does appear to benefit memory, as in the discussion of memory formation, but the specific role of dream experience is unclear.
- Emotional processing. REM sleep may help moderate the emotional charge of memories. Evidence is suggestive but contested.
- Activation-synthesis and its successors. Dreams may be the brain’s attempt to make sense of internally generated signals. Later versions of this view accept that the resulting stories can reflect the dreamer’s concerns.
- Threat or social simulation. Dreams may serve as rehearsal for dangerous or social situations, a view that has some support from content studies but limited direct testing.
- Psychoanalytic views. Freud’s idea that dreams disguise unconscious wishes was influential, but its details have not been scientifically supported.
- No function. Dreaming might be a byproduct of other processes in sleep.
Because these ideas address different levels, they are not necessarily rivals. A dream could arise from spontaneous activity, be shaped by recent memories and emotions, and have limited functional importance.
Why it is hard to resolve
Researchers cannot observe dreams directly, cannot easily run experiments that selectively remove dreaming without altering sleep itself, and must rely on imperfect reports. Studies in animals can show brain activity during REM, but not what the animal experiences. As a result, claims that science has decoded the meaning of dreams are premature. What can be said with fair confidence is that dreams are a normal part of sleep, that they often draw on waking concerns, and that their purpose remains an open question. Related topics include the broader subject of consciousness, which also lacks a settled account.