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[[Wikipedia: Neuroscience of sleep |Sleep]] is a complex phenomenon that causes physiological changes all over the body. However, it mainly affects the brain. According to the electrical activities in the brain, sleep is divided into [[Wikipedia:Rapid eye movement sleep |rapid-eye movement]] (REM) and non-REM (NREM) stages. NREM sleep is further divided into three stages according to the frequency and amplitude of brainwaves.  

[[Wikipedia: Neuroscience of sleep |Sleep]] is a complex phenomenon that causes physiological changes all over the body. However, it mainly affects the brain. According to the electrical activities in the brain, sleep is divided into [[Wikipedia:Rapid eye movement sleep |rapid-eye movement]] (REM) and non-REM (NREM) stages. NREM sleep is further divided into three stages according to the frequency and amplitude of brainwaves.  

NREM Stage 1, accounting for 5-10% of total sleep in monophasic adults, is the lightest of all NREM Stages. During this stage, awareness to the external environment and consciousness is diminished. Some people experience hypnic jerks or hypnagogic hallucinations during this stage. A mix of alpha waves and theta waves are present in this stage.

NREM Stage 1, accounting for 5-10% of total sleep in monophasic adults, is the lightest of all NREM Stages. During this stage, awareness to the external environment and consciousness is diminished. Some people experience hypnic jerks or hypnagogic hallucinations during this stage. A mix of [[Wikipedia:alpha waves|alpha waves]] and [[Wikipedia:theta waves|theta waves]] are present in this stage.

NREM Stage 2, accounting for 45-55% of total sleep, is deeper than the first one. It is more difficult to be awaken from than from the NREM 1.  This stage is characterized by K-complexes and sleep spindles, which are abrupt interruptions in alpha waves activity. This stage of sleep also plays a role in maintaining wakefulness over long periods.  

NREM Stage 2, accounting for 45-55% of total sleep, is deeper than the first one. It is more difficult to be awaken from than from the NREM 1.  This stage is characterized by [[Wikipedia:K-complexes|K-complexes]] and [[Wikipedia:sleep spindles|sleep spindles]], which are abrupt interruptions in alpha waves activity. This stage of sleep also plays a role in maintaining wakefulness over long periods.  

NREM Stage 3, accounting for 15-25% of total sleep, is the deepest of all sleep stages. This stage contains primarily delta waves, which are high-amplitude and low-frequency. Hence, this stage is also known as slow-wave sleep (SWS). It is very difficult to wake up from this stage; attempts at doing so tend to result in sleep inertia -- a period intense grogginess and impaired cognitive function. This stage of sleep plays a critical role in the maintenance of the brain through the glymphatic system. This process takes about 30 to 60 minutes of continuous NREM3 to complete, and could be hindered by schedules containing only short naps.

NREM Stage 3, accounting for 15-25% of total sleep, is the deepest of all sleep stages. This stage contains primarily [[Wikipedia:delta waves|delta waves]], which are high-amplitude and low-frequency. Hence, this stage is also known as slow-wave sleep (SWS). It is very difficult to wake up from this stage; attempts at doing so tend to result in sleep inertia -- a period intense grogginess and impaired cognitive function. This stage of sleep plays a critical role in the maintenance of the brain through the glymphatic system. This process takes about 30 to 60 minutes of continuous NREM3 to complete, and could be hindered by schedules containing only short naps.

REM, accounting for 20-25% of total sleep, is characterized by high frequency, low amplitude activity that resembles that during wakefulness and rapid eye movements. In this stage, muscles are also paralyzed and body temperature becomes unregulated. Vivid dreams are also often experienced during this stage, and the muscle atonia (paralysis) prevents the sleeper from acting them out.

REM, accounting for 20-25% of total sleep, is characterized by high frequency, low amplitude activity that resembles that during wakefulness and rapid eye movements. In this stage, muscles are also paralyzed and body temperature becomes unregulated. Vivid dreams are also often experienced during this stage, and the muscle atonia (paralysis) prevents the sleeper from acting them out.