A sleep study, known medically as polysomnography, is a non-invasive diagnostic test that monitors various bodily functions during sleep. It is often recommended by healthcare providers when there are concerns about sleep-related issues, such as excessive daytime sleepiness, chronic snoring, or unusual behaviors during the night. The primary goal of this test is to record a comprehensive overview of an individual’s sleep patterns and physiological activity to help identify or rule out sleep disorders. This article will explain what constitutes “normal” results in a sleep study, providing insight into healthy sleep architecture and physiological parameters.
Understanding Sleep Study Metrics
During a polysomnography, several types of data are collected. Electroencephalography (EEG) sensors on the scalp measure brain activity to identify sleep stages. Electrooculography (EOG) monitors eye movements, especially for rapid eye movement (REM) sleep. Electromyography (EMG) sensors track muscle activity in the chin and legs, identifying limb movements and changes in muscle tone.
Respiratory effort and airflow are also measured to detect breathing irregularities. Belts around the chest and abdomen record breathing movements, while sensors near the nose and mouth detect air movement. Oxygen levels in the blood are continuously monitored using a pulse oximeter. An electrocardiogram (ECG) records heart rate and rhythm, providing cardiac activity information.
Normal Sleep Stage Distribution
A healthy night’s sleep involves a predictable progression through different sleep stages, each with distinct brain wave patterns and physiological changes. Non-REM (NREM) sleep is divided into three stages: N1, N2, and N3. N1, light sleep, accounts for 5-10% of total sleep time. N2, a deeper NREM stage, comprises 45-55% of the night.
N3, also known as slow-wave or deep sleep, is the most restorative stage, comprising 15-20% of total sleep time. Following NREM sleep, individuals enter REM sleep, characterized by rapid eye movements, temporary muscle paralysis, and vivid dreaming. REM sleep constitutes 20-25% of the night. A normal sleep study also assesses sleep latency, the time to fall asleep, which is typically under 20 minutes for adults. Sleep efficiency, the percentage of time asleep in bed, is normal if above 85%.
Normal Physiological Readings
Beyond sleep stages, a normal sleep study report includes healthy ranges for physiological parameters. The Apnea-Hypopnea Index (AHI) or Respiratory Disturbance Index (RDI) quantifies apneas (complete pauses in breathing) and hypopneas (partial reductions in breathing) per hour. For a normal result, the AHI or RDI should be less than 5 events per hour, indicating minimal breathing disruptions.
Normal oxygen saturation levels during sleep remain above 90-92%, without significant drops. Consistent oxygen levels suggest proper respiratory function. Heart rate should exhibit normal variability without significant arrhythmias or unusual rates. Heart electrical activity is stable and within expected resting ranges.
The Periodic Limb Movement Index (PLMI) measures involuntary leg movements per hour. A normal PLMI is less than 15 per hour, indicating low frequency or absence of disruptive limb movements. Low or absent PLMI values suggest periodic limb movements are not significantly fragmenting sleep. These combined physiological measures contribute to a comprehensive understanding of sleep health.
What Normal Results Signify
Normal sleep study results indicate no significant sleep disorders were detected. This suggests an individual’s sleep architecture, including sleep stage distribution and progression, is within healthy parameters. It also implies physiological functions like breathing, oxygen levels, heart rate, and limb movements operate as expected throughout the sleep cycle.
Normal results signify healthy sleep, allowing the body and brain to undergo restorative processes without interruption. Such outcomes suggest efficient sleep that supports overall well-being. A normal sleep study provides reassurance regarding sleep health, confirming optimal nighttime physiology.