Electroencephalography, or EEG, is a foundational tool in sleep medicine, providing a non-invasive way to observe the brain’s electrical activity during the night. A sleep study, often called a polysomnogram, relies on this technology to translate the complex activity of billions of neurons into readable patterns. By tracking changes in these patterns, clinicians and researchers can objectively measure sleep quality and quantity.
How Electroencephalography Works
The EEG machine operates by detecting minute voltage fluctuations generated by large populations of neurons firing together. Small metal discs, known as electrodes, are placed at specific locations on the scalp to pick up these electrical signals. The electrodes act as passive sensors for the brain’s natural activity. Since the signals are extremely small, often measured in microvolts, they must be significantly amplified before recording.
After amplification, the raw signal is filtered to remove electrical noise and artifacts from muscle movement or external sources. The output is a continuous, graphical tracing that displays brain wave patterns over time. These tracings are characterized by two main properties: frequency (how often the waves repeat per second) and amplitude (the height or strength of the waves). The combination of frequency and amplitude provides a signature for different states of consciousness, including wakefulness and the various stages of sleep.
The Four Distinct Stages of Sleep
Sleep is divided into two primary types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is categorized into three distinct stages, beginning with the transition from wakefulness. The entire sleep cycle, which lasts approximately 90 to 110 minutes, is repeated multiple times throughout the night.
NREM Stage 1 (N1) is a light, transitional phase between being awake and falling asleep, typically lasting only a few minutes. The person then enters NREM Stage 2 (N2), which constitutes about 50% of total sleep time in adults. In N2, the body’s systems, like heart rate and breathing, slow down, and the brain shows characteristic markers that signify true sleep onset.
NREM Stage 3 (N3) is the deepest stage of sleep, often referred to as slow-wave sleep (SWS) or deep sleep. This restorative period is marked by the highest-amplitude, lowest-frequency brain waves, making it the most difficult time to awaken a person. Following deep sleep, the cycle progresses into REM sleep, a state characterized by brain activity resembling wakefulness. During REM, the eyes move rapidly beneath closed lids, and the body’s major muscle groups are temporarily paralyzed.
Identifying Key Waveforms and Markers
The specific patterns of frequency and amplitude in the EEG tracing allow sleep specialists to accurately score the four sleep stages. The transition from wakefulness into N1 is signaled by the disappearance of fast, regular alpha waves and the emergence of slower theta waves. Theta waves, which oscillate at about four to seven hertz, dominate the N1 and N2 stages.
NREM Stage 2 is uniquely defined by two transient events that interrupt the background theta activity: sleep spindles and K-complexes. Sleep spindles are brief bursts of rhythmic activity, typically in the 12 to 14 hertz range, and are thought to be involved in memory consolidation. K-complexes are sharp, high-amplitude, biphasic waveforms that appear spontaneously and can also be triggered by sudden noises or sensory stimuli.
The defining characteristic of NREM Stage 3 is the presence of delta waves, which are the slowest and largest brain waves, oscillating at less than four hertz. These high-amplitude delta waves must make up at least 20% of the EEG recording epoch for the stage to be classified as N3. REM sleep is characterized by low-amplitude, mixed-frequency activity that looks very similar to the awake state, sometimes featuring distinctive sawtooth waves often preceding bursts of rapid eye movement.
Diagnosing Sleep Disorders
The analysis of a sleep EEG tracing is the basis for diagnosing a wide range of sleep disorders by identifying deviations from a typical sleep architecture. Sleep fragmentation, characterized by frequent, brief arousals that interrupt the natural progression of stages, can be clearly identified on the EEG. These arousals prevent the brain from spending adequate time in restorative N2 and N3 sleep.
In cases of obstructive sleep apnea, the EEG reveals subtle arousals associated with breathing interruptions, which prevent the deep, continuous sleep required for daytime functioning. Narcolepsy is often diagnosed by observing sleep-onset REM periods (SOREMPs), where the patient enters REM sleep almost immediately after falling asleep, bypassing the normal NREM stages.
The lack of sufficient NREM Stage 3 sleep, or a persistent pattern of light N1 and N2 sleep, can be a marker for chronic insomnia. By quantifying the time spent in each stage and noting the presence of abnormal waveforms, the EEG provides objective data that guides treatment decisions.