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Sleep is not a single, uniform state but is composed of several distinct stages. These stages are primarily differentiated by their unique patterns of brain wave activity, which vary in frequency and amplitude. The two main phases of sleep are Rapid Eye Movement (REM) sleep and non-REM (NREM) sleep.

Stages of Sleep

Non-Rapid Eye Movement (NREM) sleep, which is divided into three stages, is a phase of sleep without rapid eye movements. The transition into NREM sleep is marked by a general shift in brain wave activity, characterized by a decrease in frequency and an increase in amplitude. Each stage within NREM sleep has its own distinct pattern of brain waves.

Non-Rapid Eye Movement (NREM) Sleep

REM sleep is a phase of sleep distinguished by the rapid movement of the eyes. Brain wave activity during this stage closely resembles that of an awake person, and it is the period when most dreaming takes place. A key feature of REM sleep is the paralysis of the body's voluntary muscle systems, although muscles essential for circulation and respiration remain active. This combination of high brain activity and lack of muscle tone is why REM sleep is often called 'paradoxical sleep'. Additionally, this sleep stage is understood to be involved in processes of learning and memory.

Rapid Eye Movement (REM) Sleep

Hypnograms show how a person passes through the various stages of sleep.

Hypnogram

Beta waves are the dominant brain wave pattern during wakefulness. Compared to the brain wave patterns observed during sleep, they are characterized by having the highest frequency (13–30 Hz) and lowest amplitude, and they also exhibit more variability.

Beta Waves

Sleep is broadly divided into two distinct types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. These categories are differentiated by physiological characteristics such as eye movement, brain wave patterns, and muscle tone. Understanding these differences is fundamental to sleep science.

Comparison of REM and Non-REM Sleep

Brain wave frequency refers to the number of brain waves that occur within a one-second interval. It is a key metric used to differentiate sleep stages and is measured in Hertz (Hz).

Brain Wave Frequency

Brain wave amplitude is the measure of the height of a brain wave. Along with frequency, it is a characteristic used to distinguish different stages of sleep and wakefulness.

Brain Wave Amplitude

An EEG-over-time graph can compare sleep-stage waveforms by showing wakefulness, Stage 1 NREM, Stage 2 NREM, Stage 3 NREM, and REM sleep on separate rows. In this example, awake activity has closely spaced waves with medium amplitude; Stage 1 NREM has relatively low-amplitude waves with periodic amplitude increases; Stage 2 NREM resembles Stage 1 but includes a K-complex around 10 to 12 seconds; Stage 3 NREM has a more uniform wave with gradually increasing amplitude; and REM resembles Stage 2 without the K-complex.

Visual Representation of Brainwaves in Sleep Stages

Brainwave activity (measured by EEG) changes dramatically across the different stages of sleep. Each sleep stage has associated wavelengths of varying amplitude and frequency.

Brainwave Activity Across Sleep Stages

A sleep researcher observes a participant's brain wave activity. At one point in the night, the readout shows low-frequency, high-amplitude waves. Later, the readout shows high-frequency, low-amplitude waves, similar to an awake state, accompanied by rapid eye movements. What is the most fundamental conclusion that can be drawn from these distinct patterns?

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