An electroencephalogram, or EEG, is a test that records the electrical signals of the brain. A sleep EEG is performed while a patient is asleep, making it a specialized tool in neurology and sleep medicine. The procedure involves placing small electrodes on the scalp to capture the brain’s electrical impulses, or brainwaves. Observing brain activity during sleep provides information about brain function that is not available while a person is awake, offering a direct look at how the brain behaves without the influence of consciousness.
The Purpose of a Sleep EEG
A primary application for a sleep EEG is the evaluation and diagnosis of seizure disorders. Some forms of epilepsy produce abnormal electrical patterns, known as epileptiform discharges, that may only become apparent when the brain transitions into a state of drowsiness or sleep. A sleep-deprived EEG, where the patient stays awake longer than usual before the test, is often used to increase the likelihood of capturing these abnormalities.
The test is also used in diagnosing a range of sleep disorders. For conditions like narcolepsy, an EEG can identify unusual patterns, such as the rapid onset of REM sleep immediately upon falling asleep, which is a characteristic feature of the disorder. For parasomnias, which are disruptive events like sleepwalking, an EEG helps correlate abnormal behaviors with specific brainwave patterns.
A sleep EEG is frequently a component of a more extensive overnight study called a polysomnogram (PSG). A PSG is the standard for diagnosing sleep-disordered breathing, including sleep apnea. While other sensors in a PSG track breathing, heart rate, and oxygen levels, the EEG portion provides the context of the sleep stages. This allows clinicians to see how events like pauses in breathing affect the brain’s sleep patterns and lead to arousals.
Understanding Brain Waves and Sleep Architecture
The electrical signals measured by an EEG manifest as brain waves, which are categorized by their frequency.
- Delta waves are the slowest and are indicative of deep, restorative sleep.
- Theta waves are slightly faster and are associated with light sleep and drowsiness.
- Alpha waves are present during relaxed wakefulness, typically when the eyes are closed.
- Beta waves are faster and signify active, alert thinking.
A night of sleep consists of several cycles through different stages, a progression known as sleep architecture. This journey begins with non-REM (NREM) sleep, which has three phases. The first stage, N1, is the transition from wakefulness to sleep, characterized on an EEG by the replacement of the waking alpha rhythm with slower theta waves. This is a light stage of sleep from which a person can be easily awakened.
Following N1, an individual enters stage N2, which constitutes the largest portion of total sleep time. The EEG during N2 is defined by two unique features: sleep spindles and K-complexes. Sleep spindles are short, rapid bursts of brain activity thought to be involved in memory consolidation, while K-complexes are large, slow waves that may help a person remain asleep in response to external stimuli.
The deepest phase of NREM sleep is stage N3, often called slow-wave sleep. This stage is dominated by high-amplitude, low-frequency delta waves. N3 is when the body performs much of its physical restoration and growth, and it is most difficult to awaken someone from this stage. The presence of significant delta wave activity is a sign of deep, restful sleep.
After cycling through the NREM stages, the brain enters REM sleep. The EEG during REM sleep is paradoxical because it shows fast-frequency waves that closely resemble the patterns of being awake. This is the stage where most vivid dreaming occurs. During REM, the body’s major muscle groups are temporarily paralyzed, a mechanism that prevents individuals from acting out their dreams. A complete sleep cycle, from N1 through REM, lasts about 90 to 110 minutes and repeats several times throughout the night.
The Sleep Study Procedure
A sleep EEG is a non-invasive and painless procedure. A technician measures the patient’s head and marks specific locations for electrode placement. These small metal discs are then attached to the scalp using a conductive paste or glue, which ensures a clear signal is transmitted to the recording equipment. Sensors may also be placed near the eyes to track eye movement and on the chin to monitor muscle tone, which helps in identifying REM sleep.
The study can be conducted in a specialized sleep laboratory or with an ambulatory device that the patient wears at home. A sleep lab provides a controlled environment designed to be quiet and comfortable. Technicians monitor the recording from an adjacent room, often using a video camera to correlate any physical movements with the recorded data. This setup is common for a comprehensive polysomnogram where breathing and heart rate are also monitored.
For an ambulatory EEG, the electrodes are secured in a clinical setting, and the patient goes home with a small, portable recording device. This allows for monitoring brain activity over 24 to 72 hours in the patient’s normal environment. While this method offers convenience, it captures less data than a full in-lab study.
Interpreting Sleep EEG Results
Once the recording is complete, a specialist analyzes the data to identify patterns and abnormalities. The raw EEG tracings are used to create a hypnogram, a graph that provides a visual summary of the entire night. This chart illustrates the progression through the different sleep stages—N1, N2, N3, and REM—allowing the clinician to see the overall structure and continuity of sleep.
From this data, several metrics are calculated to assess sleep quality. These include sleep latency, the time it takes to fall asleep, and sleep efficiency, the percentage of time spent asleep relative to the total time spent in bed. The analysis also quantifies the amount of time spent in each sleep stage. Deviations from typical percentages, such as a lack of deep N3 sleep or fragmented REM sleep, can point toward specific sleep disorders.
Clinicians also scan the EEG recordings for specific abnormal events. For individuals being evaluated for epilepsy, the focus is on identifying epileptiform activity, such as sharp waves or spikes, which are patterns indicating a propensity for seizures. In the context of sleep apnea, the EEG is examined for arousals—abrupt shifts from a deeper to a lighter stage of sleep. These arousals often occur immediately following a pause in breathing and are a primary cause of daytime sleepiness associated with the condition.