A sleep study, formally known as polysomnography (PSG), records various body functions during sleep, including brain waves, heart rate, breathing, oxygen levels, and muscle activity. Sleep specialists use the resulting technical report to diagnose conditions like sleep apnea, restless legs syndrome, and narcolepsy. Understanding the report’s acronyms and numerical indexes is the first step toward interpreting your sleep patterns. This guide will help you interpret the core measurements and structure of your sleep study results.
Deciphering the Core Metrics
The Apnea-Hypopnea Index (AHI) represents the average number of breathing events—apneas (complete cessation of airflow) and hypopneas (partial reduction in breathing)—that occur each hour of sleep. Both events must last ten seconds or longer. The AHI is the primary metric used to quantify sleep-disordered breathing, with a score below five events per hour generally considered normal.
The Respiratory Disturbance Index (RDI) provides a more complete picture of breathing issues. The RDI includes apneas and hypopneas, but also counts Respiratory Effort-Related Arousals (RERAs). RERAs are subtle breathing disruptions that cause a brief, unrecognized awakening of the brain. Because the RDI includes these milder events, its value is often higher than the AHI, revealing sleep fragmentation.
Blood oxygen content is measured by oxygen saturation (\(\text{SpO}_2\)). Healthy individuals typically maintain a saturation level between 95% and 100%, and the report notes the minimum saturation, or nadir, reached during the night. A significant drop below 90% is considered medically harmful, indicating oxygen deprivation during breathing events. The Oxygen Desaturation Index (ODI) calculates how many times per hour the blood oxygen level drops by a certain amount, usually 3% or 4%, from the baseline.
The report also includes metrics that assess the overall quality and continuity of sleep. Sleep Latency (SL) measures the time it took to fall asleep, ideally between 10 and 20 minutes. Sleep Efficiency (SE) is the percentage of time spent sleeping while in bed. A score above 80% is generally considered good for adults, while a low efficiency suggests fragmented sleep contributing to daytime fatigue.
Understanding Sleep Architecture
The body cycles through distinct phases of rest, and the breakdown of time spent in each phase is called sleep architecture. Non-Rapid Eye Movement (NREM) sleep is divided into three stages, categorized based on brain wave patterns recorded by the electroencephalogram. The N1 stage is a light, transitional state between wakefulness and sleep, typically accounting for about 5% of total sleep time.
N2 is the largest portion of sleep, making up approximately 45% to 55% of the night, and is characterized by specialized brainwave patterns called sleep spindles and K-complexes. The deepest, most physically restorative stage is N3, often referred to as slow-wave sleep. Healthy adults spend 15% to 20% of their time in N3, which is most abundant during the first half of the night.
The final stage is Rapid Eye Movement (REM) sleep, which accounts for 20% to 25% of total sleep time. This stage is associated with dreaming, muscle paralysis, and intense brain activity important for memory consolidation. Disruptions to breathing or other factors can decrease the proportion of time spent in deep N3 or REM sleep, reducing the restorative benefits. Sleep fragmentation is measured by the Arousal Index, which counts the number of brief, three-second or longer awakenings per hour of sleep.
Quantifying Sleep Disorder Severity
The AHI metric is the standard used to assign a clinical classification to Obstructive Sleep Apnea (OSA) severity. An AHI below five events per hour is considered within the normal range, indicating minimal or no sleep apnea. This severity scale provides a framework for physicians to determine the potential health risks and necessary treatment, ranging from lifestyle modifications for milder cases to continuous positive airway pressure (CPAP) therapy.
OSA Severity Classification
- Normal: Below 5 events per hour.
- Mild OSA: 5 to 14 events per hour.
- Moderate OSA: 15 to 29 events per hour.
- Severe OSA: 30 or more events per hour.
The report may also note if the disorder is position-dependent, meaning the events are significantly more frequent when sleeping on the back (supine position). This detail is important because positional therapy, such as devices that discourage back sleeping, can be an effective treatment option for some patients.
Beyond Apnea: Other Key Findings
A comprehensive sleep report includes data on neurological and motor events independent of breathing disorders. The Periodic Limb Movement Index (PLMI) measures the frequency of involuntary, repetitive limb movements, most commonly in the legs, occurring during the night. A PLMI exceeding 15 events per hour in an adult is generally considered abnormal and can be a marker for Periodic Limb Movement Disorder, which contributes to sleep fragmentation.
The study monitors heart rhythm (ECG) data, which can reveal significant cardiac events. Sleep apnea can cause rapid swings in heart rate and blood pressure, leading to arrhythmias like bradycardia (slow heart rate) or tachycardia (fast heart rate) during breathing pauses. The report highlights these abnormalities, as these findings may warrant further cardiac evaluation. Finally, the study also monitors for parasomnias, such as sleepwalking or sleep talking, which are abnormal behaviors recorded for clinical context.