The neurobiology of dreaming
Main article: Rapid eye movement sleep
EEG showing brainwaves during REM sleep
Accumulated observation has shown that dreams are strongly associated with rapid eye movement sleep, during which an electroencephalogram (EEG) shows brain activity that, among sleep states, is most like wakefulness. Participant-remembered dreams during NREM sleep are normally more mundane in comparison. During a typical lifespan, a person spends a total of about six years dreaming (which is about two hours each night). Most dreams only last 5 to 20 minutes. It is unknown where in the brain dreams originate, if there is a single origin for dreams or if multiple portions of the brain are involved, or what the purpose of dreaming is for the body or mind.
During REM sleep, the release of the neurotransmitters norepinephrine, serotonin and histamine is completely suppressed.
When REM sleep episodes were timed for their duration and subjects woken to make reports before major editing or forgetting could take place, it was determined that subjects accurately matched the length of time they judged the dream narrative to be ongoing to the length of REM sleep that preceded the awakening. There is no “time dilation” effect; a five-minute dream takes roughly five minutes of real time to play out.This close correlation of REM sleep and dream experience was the basis of the first series of reports describing the nature of dreaming: that it is a regular nightly, rather than occasional, phenomenon, and a high-frequency activity within each sleep period occurring at predictable intervals of approximately every 60–۹۰ minutes in all humans throughout the life span.
REM sleep episodes and the dreams that accompany them lengthen progressively across the night, with the first episode being shortest, of approximately 10–۱۲ minutes duration, and the second and third episodes increasing to 15–۲۰ minutes. Dreams at the end of the night may last as long as 15 minutes, although these may be experienced as several distinct stories due to momentary arousals interrupting sleep as the night ends. Dream reports can be reported from normal subjects on 50% of the occasion when an awakening is made prior to the end of the first REM period. This rate of retrieval is increased to about 99% when awakenings are made from the last REM period of the night. This increase in the ability to recall appears related to intensification across the night in the vividness of dream imagery, colors, and emotions.
Dreams in animals
REM sleep and the ability to dream seem to be embedded in the biology of many organisms that live on Earth. All mammals experience REM. The range of REM can be seen across species: dolphins experience minimum REM, while humans remain in the middle and the opossum and the armadillo are among the most prolific dreamers.
Studies have observed dreaming in mammals such as monkeys, dogs, cats, rats, elephants and shrews. There have also been signs of dreaming in birds and reptiles. Sleeping and dreaming are intertwined. Scientific research results regarding the function of dreaming in animals remain disputable; however, the function of sleeping in living organisms is increasingly clear. For example, recent sleep deprivation experiments conducted on rats and other animals have resulted in the deterioration of physiological functioning and actual tissue damage of the animals.
Some scientists argue that humans dream for the same reason other amniotes do. From a Darwinian perspective dreams would have to fulfill some kind of biological requirement, provide some benefit for natural selection to take place, or at least of have no negative impact on fitness. In 2000 Antti Revonsuo, a professor at the University of Turku in Finland, claimed that centuries ago dreams would prepare humans for recognizing and avoiding danger by presenting a simulation of threatening events. The theory has therefore been called the threat-simulation theory. According to Tsoukalas (2012) dreaming is related to the reactive patterns elicited by predatorial encounters, a fact that is still evident in the control mechanisms of REM sleep .