What is a dream?
Dreaming is a mental activity that occurs during sleep. Dream memory varies greatly from one sleeper to another: some people report very few dreams, or even think that they are not dreaming, while others report having several dreams per night. On average, it is reported in normal population, one to two dreams per week.
The dream includes several phases: a sensory production (visual, auditory, olfactory, tactile…) which is stored in memory and transformed to become a story. What is called a dream therefore results from different cerebral processes and involves complex cognitive functions such as memory. The study of dreams with scientific methods is recent and inseparable from the study of sleep: the discovery of REM sleep in the 1950s was a major breakthrough in dream research. In sleep recording laboratories, the observation of certain disorders associated with intense dreams, such as night terrors or somnambulism, or even restless dreams called "REM sleep behavior disorders", has also brought insights over the past twenty years, on the mechanisms of sleep and dreaming. Researchers have also equipped themselves with questionnaires and analysis grids to study dream stories in the general population and veritable "dream banks" grouping together thousands of nocturnal stories allow them to access samples of varied and numerous ; the best known is the dream bank of the American psychologist, William Domhorff. Finally, research laboratories now have functional cerebral imaging tools that provide more and more knowledge on mental processes during sleep, including the formation of dreams.
Currently it is not so much the interpretation of the meaning of dreams that animates the scientific debate, as the question of the functions of dreams and their relationship with consciousness.
When do we dream?
With the discovery of REM sleep, which includes high electrical activity and rapid eye movements, this stage of sleep was long thought to be the container of dreaming; especially since people awakened during REM sleep report a dream memory in 80 to 90% of cases. Studies have then shown that we have the ability to dream throughout the night but that our dreams do not have the same intensity or the same nature during the different stages of sleep. During slow wave sleep, it is mainly thoughts that come to mind or factual dreams, not very imaginative, closer to daily concerns than to phantasmagorical constructions. During REM sleep, mental activity appears more intense with more numerous dream accounts, more scripted, longer, richer in emotions, and of often bizarre or striking content.
The hypothesis according to which the dream would be built instantaneously at the time of awakening is contradicted by the observation in sleep laboratories of certain disorders such as somnambulism or restless dreams during REM sleep called "REM sleep behavior disorders". » ; the video recording showed that oneiric behaviors could last for long minutes and be consistent with the dream narrative made by the subject upon waking. The dream is therefore not only an instantaneous construction linked to waking up. Thanks to precise recording techniques (high density electroencephalogram), periods of dream production are sought from variations in the level of brain wave activity during sleep.