Polysomnography | What Really Happens in Your Brain While You Sleep?

Polysomnography (PSG) is the gold-standard method for evaluating sleep. It measures various physiological activities to precisely identify sleep stages. The core of this process is the electroencephalogram (EEG), which records the brain's electrical activity via sensors placed on the scalp. Different sleep stages are characterized by distinct brainwave patterns. For instance, deep sleep (NREM Stage 3) is identified by slow, high-amplitude delta waves, indicating a state of deep rest and restoration. In contrast, Rapid Eye Movement (REM) sleep, where most dreaming occurs, shows brain activity that closely resembles the waking state, with faster, low-amplitude waves. The test also uses an electrooculogram (EOG) to track eye movements—distinguishing the rapid, darting eyes of REM from the slow, rolling movements of early NREM sleep. Finally, an electromyogram (EMG) measures muscle tension, typically under the chin. During REM sleep, the body experiences a temporary, protective paralysis called atonia to prevent acting out dreams, which is clearly visible as a significant drop in EMG activity. By integrating data from the EEG, EOG, and EMG, clinicians can construct a detailed map of a person's sleep architecture, charting the progression through each stage over the course of the night.

Beyond the core brain and muscle monitoring, a comprehensive PSG tracks a wide array of bodily functions to obtain a holistic view of sleep health. An electrocardiogram (EKG) records heart rate and rhythm, identifying any cardiac irregularities that may occur during sleep. A pulse oximeter, usually clipped to a fingertip, measures blood oxygen saturation levels, which is critical for diagnosing respiratory issues like sleep apnea where oxygen levels can drop. Respiratory effort is monitored using elastic belts around the chest and abdomen to track breathing movements. Airflow is measured with a sensor placed near the nose and mouth. Together, these tools can detect apneas (complete cessation of breathing) and hypopneas (shallow breathing). Additionally, sensors on the legs record limb movements to identify conditions such as Periodic Limb Movement Disorder (PLMD). Audio recording is also common to detect snoring or other unusual sounds. All these signals provide a complete, multi-faceted picture of physiological function during sleep.

The PSG test is a non-invasive and painless procedure. It does not involve any needles or injections. The monitoring is conducted using surface electrodes and sensors that are attached to the skin with a mild adhesive or medical tape. While having these sensors attached may feel unfamiliar, it is not designed to be painful. Sleep technicians are trained to apply them in a way that minimizes discomfort to ensure the patient can sleep as naturally as possible. Most individuals acclimate to the sensors quickly and are able to sleep sufficiently for a valid study. The goal is to observe typical sleep patterns, so patient comfort is a priority.

PSG is the definitive diagnostic tool for a wide range of sleep disorders. Its most common application is for sleep-disordered breathing, primarily Obstructive Sleep Apnea (OSA), where it can quantify the frequency and severity of respiratory events and associated oxygen desaturation. It is also essential for diagnosing central nervous system disorders of hypersomnolence, such as narcolepsy, by identifying characteristic abnormalities like sleep-onset REM periods (entering the dream stage almost immediately after falling asleep). Furthermore, PSG can identify parasomnias, which are disruptive sleep-related events like sleepwalking, REM sleep behavior disorder (acting out dreams), or sleep terrors. It is also used to diagnose Periodic Limb Movement Disorder (PLMD) by detecting repetitive leg movements that cause sleep fragmentation.

After the overnight study is complete, a trained sleep technologist scores the voluminous raw data, and a sleep medicine physician interprets the results. The final report includes a graphical representation of the night's sleep called a hypnogram, which visually charts the progression through different sleep stages. The specialist analyzes the "sleep architecture," which includes metrics like total sleep time, sleep efficiency (time asleep divided by time in bed), and the percentage of time spent in each sleep stage (N1, N2, N3, REM). The report quantifies any abnormalities observed, such as the Apnea-Hypopnea Index (AHI), which is the number of breathing events per hour, the Oxygen Desaturation Index (ODI), and the number of arousals or awakenings. By synthesizing all this information, the physician can identify specific sleep disorder patterns, determine the severity of the condition, and formulate an appropriate treatment plan. For instance, a high AHI with corresponding oxygen drops confirms sleep apnea, while frequent leg movements followed by arousals would point to PLMD.