In humans, sleep can be categorized into two types: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. The NREM type is further subdivided into three more sleep stages: N1, N2 and N3. It is important to clarify that sleep stages do not occur in a sequence; in fact, the N2 stage is repeated before REM sleep takes place. The specific order for the sleep stages cycle is normally N1-N2-N3-N2-REM.
Stage N1: this is the beginning of the sleep cycle, and commonly involves what people refer to as "light sleep". In fact, this stage could be defined more precisely as the transition between wakefulness and sleep. The brain slows down and goes from producing alpha waves to theta waves. Myoclonus (sudden jerks) are common during this stage, and the individual may have at least some awareness of the environment. In other words, the N1 stage refers to shallow sleep.
Stage N2: this stage is characterized by the onset of "sleep spindles", which consist of rapid bursts of brain activity. Muscular activity decreases and any awareness of external environment is no longer present. This stage accounts for about 50% of total sleep in adults.
Stage N3: this stage is known as "deep sleep", and involves the presence of high amplitude delta waves. The onset of parasomnias (like night terrors, sleepwalking or bedwetting) is linked to this stage.
REM sleep: this is where most dreaming occurs. It accounts for approximately 25% of total sleep. This stage includes rapid eye movement, increased brain activity and increased respiration rate. The body also achieves muscular atonia (muscles become paralyzed). Scientists have claimed that the onset of this paralysis has the function of protecting the individual from self inflicted damage through physically acting out dreams.
It is important to note that these different sleep stages not only occur on humans, but on the rest and mammals and birds as well.
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