A dream is a sequence of images, often highly graphic, sometimes with sounds, smells and other sensations, that usually occur in the mind during certain stages of sleep. The meaning and purpose of dreams are not fully understood by the science. The scientific study of dreams is called oneirology.
There are numerous theories of dreams, such as:
- Robert (1886), was the first who suggested that dreams have the important function to erase impressions that were not fully worked up, and ideas that were not fully developed during the day. By the dream work, incomplete material is either removed (suppressed) or deepened and included into memory. Hughlings Jackson (1911) also viewed that sleep serves to sweep away unnecessary memories and connections from the day.
- In the late 19th century Sigmund Freud developed a theory that the content of dreams is driven by unconscious desires. Later he concluded that trauma or aggression could influence dream content.
- In 1976 J. Allan Hobson and Robert McCarley suggested that dreams are simple attempts of the brain to make sense of various signals it experiences during rapid eye movement sleep.
- Jie Zhang proposed that the function of sleep is to process, encode, and transfer the data from the temporary memory store to the long-term memory store. So the dreaming is self-maintained process of the dreamer's own thinking until the next pulse of memory insertion.
- Evolutionary psychologists believe dreams serve some adaptive function for survival. Deirdre Barrett thinks people continue to work on all the same problems – personal and objective – while dreaming, and any dilemma may get solved through having dreams.
- Hartmann says dreams may function like psychotherapy, by "making connections in a safe place" and allowing the dreamer to integrate thoughts that may be disturbing in usual state.
Dreams mainly occur in the rapid-eye movement (REM) stage of sleep, when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep.
The length of a dream can vary; they may last for a few seconds, or 20–30 minutes. People are more likely to remember the dream if they are awakened during the REM phase. The average person has three to five dreams per night, and some may have up to seven. However, most dreams are immediately or quickly forgotten.