Polysomnography (PSG) is a diagnostic tool used to record multiple physiological parameters while a person sleeps, providing an objective snapshot of sleep health. This overnight study monitors brain waves, eye movements, muscle activity, heart rhythm, breathing patterns, and blood oxygen levels to identify disruptions that may indicate a sleep disorder. The resulting report contains numerous data points that reveal the overall quality and architecture of a night’s rest. Understanding the normal thresholds for these metrics is the first step in making sense of a sleep study report.
The Stages and Cycles of Restorative Sleep
A healthy night of rest is defined by a predictable pattern of cycling through different sleep stages, known collectively as sleep architecture. Sleep is broadly divided into two states: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. A complete cycle lasts approximately 90 to 120 minutes, and a typical adult completes about four to five cycles over a full night of sleep.
NREM sleep is further broken down into three stages. N1 is the lightest stage, a transitional phase between wakefulness and sleep, typically accounting for about five percent of total sleep time. The N2 stage is considered true physiological sleep and is the most abundant, making up about 45 to 55 percent of the night. It is characterized by specific brain wave patterns like sleep spindles and K-complexes.
The deepest and most restorative sleep occurs during the N3 stage, also called slow-wave sleep. This stage is characterized by slow, high-amplitude delta waves in the brain and normally accounts for 15 to 20 percent of total sleep time in adults. Sufficient N3 sleep is necessary for physical recovery and growth hormone release, and it predominates in the first half of the night.
The final stage is REM sleep, which typically occupies 20 to 25 percent of the night and is associated with dreaming, memory consolidation, and high brain activity. During REM, the body experiences a temporary paralysis of major muscle groups (atonia), while the eyes move rapidly beneath the eyelids. As the night progresses, REM periods lengthen, while the amount of N3 sleep decreases.
Essential Metrics for Overall Sleep Efficiency
Beyond the percentages of time spent in each stage, a sleep study report quantifies the overall timing and effectiveness of the sleep period. Sleep Latency is the time it takes to transition from being awake to the first epoch of sleep. For a healthy adult, a normal sleep latency is considered to be between 10 and 20 minutes.
Falling asleep much faster than this (less than eight minutes) can suggest excessive sleepiness or a significant sleep debt. Conversely, a prolonged sleep latency of greater than 30 minutes may indicate difficulty initiating sleep, often associated with insomnia.
Sleep Efficiency is calculated as the percentage of time spent asleep relative to the total time spent in bed. A score of 85 percent or higher is generally considered a good result for adults, indicating effective use of the time designated for sleep. A lower percentage suggests significant time spent awake after falling asleep or a long delay in sleep onset.
The Total Sleep Time (TST) records the overall duration of time spent in all sleep stages during the study. While TST is variable depending on individual needs, the general recommendation for most adults is to achieve between seven and nine hours of sleep per night. TST provides context for the other percentage-based metrics when analyzing a sleep study.
Interpreting Respiratory and Movement Indices
The most frequently referenced metrics relate to physical events that disrupt breathing or cause physical movement. The Apnea-Hypopnea Index (AHI) is a primary measure, calculated by dividing the total number of apneas and hypopneas by the total sleep time in hours. An apnea is recorded when breathing stops completely or nearly completely for ten seconds or more, while a hypopnea is a partial reduction in airflow.
A normal AHI score for an adult is less than five events per hour. This threshold signifies that breathing disruptions are minimal and do not meet the criteria for diagnosing sleep-disordered breathing. A score of zero is rare, and a few events per hour are expected even in healthy individuals.
The Respiratory Disturbance Index (RDI) is a broader measure that includes apneas and hypopneas, along with Respiratory Effort-Related Arousals (RERAs). RERAs are periods of increased breathing effort that lead to an arousal from sleep without meeting the technical criteria for an apnea or hypopnea. Because RDI includes RERAs, the RDI score is always equal to or higher than the AHI score.
The RDI helps capture more subtle breathing issues that fragment sleep, and its normal threshold is also generally considered to be less than five events per hour. A high RDI with a low AHI suggests upper airway resistance that is not severe enough to cause full apneas or hypopneas.
Finally, the Periodic Limb Movement Index (PLMI) assesses the frequency of repetitive, involuntary muscle twitches, typically in the legs. These movements must occur in a series of at least four, spaced close together, and are counted only if they cause an arousal from sleep. A normal result is generally considered to be less than 15 movements per hour of sleep. A score above this threshold may indicate Periodic Limb Movement Disorder, which can contribute to restless or non-restorative sleep even without a high AHI score.