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  • Writer's pictureJames Waring

Your sleep study results explained

If you're struggling with a sleep disorder, such as sleep apnea, your physician may recommend a sleep study test to aid in the diagnosis of the underlying condition. After taking such a test, you may be wondering what the results say and what that means for you and your sleep health.

The results of your sleep study can often be confusing. There are also several types of sleep studies conducted depending on the underlying condition and your physician's preference. Each sleep study test provides a different set of results. That’s why it’s essential to understand which type of sleep study you received and what the results mean for you.

Let’s look at the information you should know to understand your sleep study results.

Types of sleep studies

The number of different sleep studies used to diagnose sleep disorders, such as sleep apnea, has increased in recent years. What began as a complex, expensive test conducted only in the hospital has shifted towards simpler, faster, at-home options today.

The type of sleep study your physician recommends depends on the complexity of your personal health history and the suspected cause of your sleep disorder. These studies are divided into the following categories depending on the types of measurements taken and the presence of a healthcare professional during the test.

Let’s take a closer look at each type, starting with the Type I sleep study.

Photo by Somnomedics

Type I sleep study

A Type I sleep study, also known as polysomnography or PSG, measures your brain, cardiovascular, respiratory, and muscle activity while you sleep. Considered the “gold standard” of sleep testing, these tests are conducted overnight in a specialized facility under the supervision of a trained sleep technologist. You can read more about a Type I sleep study from the manufacturer Somnomedics here.

Photo by Nox Medical

Type II sleep study

Type II sleep studies measure the same activity as a Type I sleep study and are also considered polysomnography. The primary difference is a Type II sleep study is conducted at your home rather than in a specialized sleep testing facility. You can read more about a Type II sleep study from the manufacturer Nox Medical here.

Photo by ResMed

Type III sleep study

A Type III sleep study, also known as a home sleep test (HST or HSAT), is a more straightforward test that measures your respiratory and cardiovascular activity and, in some cases, your body movement as well. These tests are also taken at your home rather than a sleep facility. You can read more about a Type III sleep study from the manufacturer ResMed here.

Photo by Itamar Medical

Type IV sleep study

A Type IV sleep study is another form of HST that uses even fewer sensors than the Type III sleep study. Rather than directly measuring your respiratory activity, these devices calculate it from the measurement of your cardiovascular activity. You can read more about a Type IV sleep study from the manufacturer Itamar Medical here.

Photo by SleepScore

Sleep screener

In addition to the sleep study tests described above, there are an increasing number of screening tests available to help you understand your risk of sleep apnea. These range from clinically validated questionnaires to smartphone apps that monitor your sleeping. While none of these screeners can be used to diagnose sleep apnea, they can be helpful to understand your risk and gather more information. And the best part is many of these are available for free online! Some popular screeners include:

  • STOP-BANG questionnaire

  • NoSAS questionnaire

  • Berlin questionnaire

  • SleepScore Labs app

  • SnoreLab app

Now that you’ve seen the different types of sleep study tests and screeners available, we’ll head to the next section to discuss what information is collected during these tests and how it is used.

Sleep study measurement

Each sleep study described above records different signals from your body while you sleep. Your physician uses these measurements to understand better what is happening during your sleep and aid in diagnosing a sleep disorder.

Often using multiple sensors, these sleep studies collect information about the primary organ systems involved in sleep: respiratory, cardiovascular, muscular, and nervous. These sensors can be grouped into four categories based on the activity they measure.

  1. Respiratory activity

  2. Cardiovascular activity

  3. Brainwave activity (nervous system)

  4. Muscle activity

We’ll look at each category closer, starting with measuring your respiratory activity.

Photo by Jair Lázaro on Unsplash

Respiratory activity

The primary measure in any sleep study which diagnoses sleep apnea is the amount and type of respiratory events you have while you sleep. There are several events a test will measure depending on the kind of test:

  • Apnea: an event where your airway completely closes, and there is no airflow for 10 seconds or longer. Apneas are of two types: obstructive (airway physically blocked) or central (airway open).

  • Hypopnea: an event where your airway partially closes and significantly reduces airflow for 10 seconds or longer.

  • Respiratory Effort Related Arousal (RERA): an event where airflow is reduced and followed by increased breathing effort leading to an arousal from sleep.

The most commonly reported measure in a sleep study is the Apnea-Hypopnea Index or AHI. The AHI is calculated by dividing the total number of apneas and hypopneas you have while sleeping by the total number of hours slept. If the number of RERAs is also included in the total, the measure becomes the Respiratory Disturbance Index or RDI. Your AHI is the primary measure by which a physician diagnoses sleep apnea. We will discuss the different AHI levels is in the next section.

Cardiovascular events

Another commonly reported measure from a sleep study is the behavior of your cardiovascular system while you sleep. Most tests will measure your heart rate, or pulse, which is the number of times your heart beats during a single minute. Sudden changes in your heart rate can indicate a respiratory event is occurring.

In addition to your pulse, your oxygen saturation or oxygen level present in your bloodstream is also reported. This measure is shown as a percentage from 0 to 100. Normal levels for a healthy adult are typically in the mid to high 90s. When you stop breathing during sleep, the oxygen level in your blood begins to fall, and you experience a desaturation event. The number of desaturations while sleeping divided by the number of hours slept in a sleep study is reported as your Oxygen Desaturation Index or ODI.

Brainwave activity

If you take a Type I or Type II sleep study, you will have additional sensors measuring your brainwave activity. Called an electroencephalogram or EEG, this test measures the electrical activity in your brain. Doing so allows your physician to measure several sleep characteristics that help confirm a diagnosis of sleep apnea or other sleep disorders. These include:

  • Sleep Stage: the stage or cycle of your sleep includes light sleep (N1 stage), deep sleep (N2 & N3 stages), and rapid-eye-movement or REM sleep.

  • Sleep Latency: the amount of time it takes your brain to fall asleep once you go to bed.

  • Arousal Index or AI: the total number of times you wake up divided by the total number of hours slept during the sleep study.

  • Sleep Efficiency: the amount of time your brain is awake while in bed relative to the amount of time you are asleep.

Measuring your brainwave activity allows your physician to determine the impact your sleep staging has on your sleep apnea. Some respiratory events can be tied to specific sleep stages, such as REM sleep. This measurement also provides a highly accurate assessment of your sleep quality and length.

Muscle activity

In addition to brainwave activity being measured during a Type I or Type II sleep study, your muscle activity can also be measured using electromyography or EMG. By measuring this activity, your physician can determine whether you suffer from uncontrolled limb movement ( Restless Leg Syndrome or RLS), sleepwalking or parasomnia, or motion during REM sleep. Body position can also be measured to determine if the severity of respiratory events is linked to specific body positions, such as lying on your back or stomach.

Measuring the activity from multiple organ systems helps provide a complete picture of what is happening to your body while you sleep. Your physician will review these measurements to understand your sleep disorder and confirm a diagnosis.

Now that we’ve reviewed what information is collected during a sleep study, we’ll look at what the results of a sleep study can mean for you and your sleep health.

Photo by Nik Shuliahin on Unsplash

Sleep study results

After completing your sleep study, your physician will review the results and assess the measurements taken during your sleep. If a diagnosis of sleep apnea is suspected, the primary results reviewed are your AHI or RDI as well as your ODI.

By looking at these two measurements, your physician can understand how often you suffer from respiratory events while you sleep and the resulting impact on your cardiovascular system. Depending on the score for your AHI, your diagnosis will fall into one of four categories.

Let’s explore each result further, starting with a normal result.


No diagnosis of sleep apnea will be confirmed when your AHI falls below five events/hour throughout the test. Though you may have occasional respiratory events while you sleep, this is normal and quite common.

Even if your sleep study results come back normal, there is a possibility you may have a different condition called upper airway resistance syndrome or UARS. Like sleep apnea, the tissue surrounding the airway collapses during sleep, causing the airway to narrow and the airway resistance to increase. This increased resistance is enough to increase the breathing effort and cause arousals. These repeated arousals lead to disturbed sleep and similar symptoms to sleep apnea. However, the airway collapse is not enough to meet the criteria for a sleep apnea diagnosis.

Mild sleep apnea

A diagnosis of mild sleep apnea is confirmed when your AHI falls between 5 and 15 events/hour throughout the night. Many of the severe side effects associated with untreated sleep apnea have yet to be linked to untreated mild sleep apnea. However, mild sleep apnea sufferers may experience the symptoms of severe cases such as morning headaches and daytime sleepiness.

Photo by Kate on Unsplash

Moderate sleep apnea

A diagnosis of moderate sleep apnea is confirmed when your AHI falls between 15 and 30 events/hour throughout the night. In addition to the daytime symptoms, moderate sleep apnea can have a severe impact on your long-term health when left untreated. Untreated moderate sleep apnea has been linked to other conditions such as diabetes, high blood pressure, and stroke.

Severe sleep apnea

A diagnosis of severe sleep apnea is confirmed when your AHI falls above 30 events/hour throughout the night. Like moderate sleep apnea, severe sleep apnea has severe side effects when left untreated. Often severe sleep apnea sufferers experience such intense symptoms it disrupts their daily life.

Your sleep apnea can also be dependent on your sleeping position or a specific sleep stage. Information gathered from some of the more sophisticated tests can aid in determining this. This insight can be helpful to determine which sleep apnea treatment is best for you.

Depending on the type of sleep study conducted, additional information surrounding your diagnosis may be available. If you completed a Type I or Type II sleep study, other sleep disorders such as restless leg syndrome, insomnia, and parasomnia might also be diagnosed. It is possible to suffer from multiple sleep disorders simultaneously, such as sleep apnea and insomnia.

Photo by Gia Oris on Unsplash

What To Do With Your Results

Regardless of your diagnosis, it’s important to take sleep apnea seriously. In addition to the daily symptoms it can cause, sleep apnea has deadly side effects when left untreated. If you have recently been diagnosed with sleep apnea, you should consider the many sleep apnea treatments available to you. Consult your doctor on your sleep study results and discuss the treatment options available.

The most common sleep apnea treatment is a CPAP (continuous positive airway pressure) machine. Alternative therapies include an oral appliance, upper airway stimulation, positional therapy, and surgery, among other options.

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