Sleep scoring analyzes physiological data to classify sleep into distinct stages and identify specific events. This process creates a detailed map of an individual’s sleep architecture, which is used to understand sleep quality and identify patterns related to various sleep disorders.
Gathering Sleep Data for Analysis
To analyze sleep, specialists require detailed physiological data collected through an overnight test called polysomnography (PSG). This study is conducted in a sleep laboratory, where multiple sensors are attached to the body to record activity throughout the night. This setup ensures that the data is high-quality and free from artifacts that could interfere with interpretation.
The core of a PSG study involves three main types of sensors. An electroencephalogram (EEG) measures brain wave activity using small electrodes placed on the scalp. An electrooculogram (EOG) records eye movements. An electromyogram (EMG) monitors muscle activity, with sensors placed on the chin to detect muscle tone and on the legs to identify movements.
Beyond these, other sensors provide a more complete picture of sleep health:
- An electrocardiogram (ECG) tracks heart rate and rhythm.
- Respiratory sensors monitor airflow through the nose and mouth.
- Elastic belts around the chest and abdomen measure respiratory effort.
- A pulse oximeter clipped to a fingertip tracks blood oxygen levels.
Identifying Different Sleep Stages
The data from polysomnography allows scorers to distinguish between wakefulness and the various sleep stages. Before sleep, a state of wakefulness is characterized by active brain waves, frequent eye blinks, and high muscle tone on the EEG, EOG, and EMG recordings.
Sleep is divided into two types: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM). NREM sleep has three stages (N1, N2, and N3) that progress into deeper sleep. A typical night involves cycling through NREM and REM stages multiple times, with each cycle lasting 90 to 110 minutes.
Stage N1 is the transition into light sleep, where a person is easily awakened, brain waves slow, and the EOG may detect slow eye movements. Stage N2 is a deeper phase that accounts for the largest portion of sleep in most adults. It is defined by specific brainwave patterns called sleep spindles and K-complexes.
Stage N3, or deep sleep, is characterized by high-amplitude, slow delta waves on the EEG. This is the most restorative stage, during which the body undergoes repair and growth. It is followed by the progression to REM sleep.
During REM sleep, the EEG shows brain activity similar to wakefulness. The EOG detects rapid eye movements, while the EMG shows a near-total paralysis of most muscles, a state known as atonia. This stage is also most associated with vivid dreaming.
The Manual Scoring Process
The analysis of sleep data is performed by trained sleep technologists who visually inspect the night’s recordings. They break the data into 30-second segments called “epochs.” Each epoch is then assigned a sleep stage (Wake, N1, N2, N3, or REM) based on the combined signals from the EEG, EOG, and EMG.
This classification follows standardized criteria from resources like the American Academy of Sleep Medicine (AASM) Manual. This guide provides precise rules for identifying the start and end of each sleep stage and for flagging other events, such as arousals or respiratory disturbances. Following these rules ensures scoring is consistent across different labs.
Scorers must distinguish subtle shifts in waveform patterns that define the boundaries between stages. To maintain accuracy, sleep centers assess inter-scorer reliability by having multiple technologists score the same record. While automated scoring software is becoming more common, manual scoring by an expert remains the benchmark for accuracy.
Key Metrics Derived from Sleep Scoring
The scored data is compiled into a summary of metrics about an individual’s sleep. One primary metric is Total Sleep Time (TST), which is the cumulative amount of time spent in any sleep stage. This figure gives a basic measure of sleep duration for the night.
Sleep Efficiency is the percentage of time spent asleep relative to the total time in bed. For example, a sleep efficiency of 90% means that for eight hours in bed, just over seven hours were actual sleep. Other metrics include Sleep Latency, the time it takes to fall asleep, and REM Latency, the time from sleep onset to the first REM period.
The report details the percentage of time spent in each sleep stage (N1, N2, N3, and REM). A balanced distribution across these stages indicates healthy sleep architecture, as sufficient deep sleep (N3) and REM sleep are needed for restoration. Frequent arousals or awakenings, which are also tallied, can fragment sleep and reduce its quality even if total sleep time seems adequate.