As soon as we sleep, a change in brain activity and chemistry begins to occur in parallel with a dream-like mental activity. Consciousness is already beginning to change at the very beginning of sleep. As sleep deepens, cerebral energy metabolism and neural activity related to blood flow begin to decrease, especially during non-REM sleep, the deepest phase of sleep. During these deep stages, the connection between brain regions is reduced and local connections remain more active. The situation experienced by people under general anesthesia is actually similar. All external senses are turned off and awareness is lowered to very low levels. Long distance connections are for consolidating information and enabling communication between brain areas.
As we come out of deep sleep, brain waves also begin to change and are increasingly similar to our awake brain waves. This stage, which is the part where sleep is dreamed and covers approximately one third of sleep, is called REM sleep. When exiting the deep sleep phases that make up two of the 4 phases of sleep, not all regions of the brain are reactivated. Therefore, a selective activation is applied to regions of the brain in the transition from deep sleep to REM sleep. The reactivation of the brain regions alters the short-term autobiographical memory, affecting dreaming and the content of dreams. For example, if we have experienced an emotional event during the day, our limbic, paralimbic and amygdala regions are activated during REM sleep. Another region that is activated in REM sleep is the medial prefrontal cortex. This part of the cortex is responsible for producing slow sleep waves, and is a very active area when we think about the behavior of others.