Accumulated observation has shown that dreams are strongly associated with REM 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.
During most dreams, the person dreaming is not aware that they are dreaming, no matter how absurd or eccentric the dream is. The reason for this may be that the prefrontal cortex, the region of the brain responsible for logic and planning, exhibits decreased activity during dreams. This allows the dreamer to more actively interact with the dream without thinking about what might happen, since things that would normally stand out in reality blend in with the dream scenery.
When REM sleep episodes were timed for their duration and subjects were awakened to make reports before major editing or forgetting of their dreams could take place, subjects accurately reported the length of time they had been dreaming in an REM sleep state. Some researchers have speculated that "time dilation" effects only seem to be taking place upon reflection and do not truly occur within dreams. 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 is correlated with high-frequency activity within each sleep period occurring at predictable intervals of approximately every 60–90 minutes in all humans throughout the lifespan.
REM sleep episodes and the dreams that accompany them lengthen progressively through the night, with the first episode being shortest, of approximately 10–12 minutes duration, and the second and third episodes increasing to 15–20 minutes. Dreams at the end of the night may last as long as 15 minutes, although these may be experienced as several distinct episodes due to momentary arousals interrupting sleep as the night ends. Dream reports can be reported from normal subjects 50% of the time when they are awakened 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. The increase in the ability to recall dreams appears related to intensification across the night in the vividness of dream imagery, colors, and emotions.
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