Interpreting Sleep Study Results – Polysomnography (PSG)

Your primary care provider recommends that you undergo a PSG (Polysomnography). What does this mean, and how do you go about interpreting sleep study results?

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This article in short:

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  • A Sleep Study (polysomnography) is recommended when your sleep complaints raise the suspicion of sleep apnea.
  • During a sleep study, specialists measure and record several of your biological functions while you sleep.
  • A Sleep Study can take place in your home or a laboratory.
  • The AHI is the first number you should check on your sleep report.
  • Your physician will interpret the results and recommend treatment if needed.

What is a Sleep Study?

A Polysomnography test is what is known in more trivial terms as a “Sleep Study.” To undergo such a test, you may have to go to a sleep clinic and spend a night there. While sleeping, you will be hooked up to various sensors that will record certain aspects of your biological functions as you sleep.

In the morning, the study produces a set of results. Specialists use these results to determine whether you suffer from some type of sleep disorder.

The reason your physician may recommend such a study is that he/she suspects that you may have a sleep disorder. The spectrum of likely ailments includes the following:

  • Sleep apnea, or some type of breathing-related sleep disorder. Sleep apnea sufferers will stop breathing for several seconds at a time during sleep. They then wake up struggling for breath. The condition is extremely dangerous and it is detrimental to health on several levels.
  • Narcolepsy. Those who suffer from this condition experience intense bouts of daytime sleepiness. Many do indeed yield to the urge to sleep. The dangers of narcolepsy are obvious.
  • Unexplained insomnia. If your doctor cannot pinpoint a likely cause for your insomnia, he/she may resort to a sleep study to set a course for treatment.
  • REM sleep behavior disorder. If you act out your dreams while asleep, you may be suffering from this disorder. This disorder ruins sleep and entails other dangers for the sufferer.
  • Various other out-of-the-ordinary, in-sleep behaviors. If you rock yourself to sleep and continue your rhythmic movement during sleep or sleepwalk, you fall into this category.

Sleep disorders used to be largely overlooked as health-threatening conditions in the past. Lately, there have been more and more requests for sleep studies from health care providers.

What Does a Sleep Study Entail?

During a sleep study, the specialist will measure several biological indicators of sleep quality. He/she will gather quantitative data about symptoms and technical details. Sleep architecture is also studied. This part of the research is focused on the distribution of various sleep stages.

To complete the measurements, the following equipment may be used:

How to go about Interpreting Sleep Study Results?

A Sleep Study will usually start with information about the patient. This part is as easy to understand as it gets.

Past that, the first result you should consider is the AHI or RDI. The Apnea/hypopnea Index or the Respiratory Disturbance Index offers a bird’s eye view of the study. This number does a good job of describing your sleep situation, in and of itself.

The AHI is defined as the sum of apnea- and hypopnea-triggered events per hour.

Here is what you need to know in its regard:

  • An AHI<5 means that you do not suffer from a sleep disturbance.
  • An AHI value in the 5-15 range indicates mild sleep apnea.
  • If your AHI is in the 15-30 range, you suffer from moderate sleep apnea.
  • Anything above 30 means severe sleep apnea.

What is the RDI? We can define the RDI as the sum of all apneas, hypopneas, and RERAs, per total sleep time. A RERA is a sleep event triggered by gradually higher respiratory effort, which results in awakening. A RERA does not count as apnea or hypopnea.

Let us take a systematic look at the variables that make up your AHI, your RDI, and your sleep report.

  • The Total Respiratory Events entry is self-explanatory. It covers all apneas, hypopneas, and RERAs.
  • We define Central Apneas as apneas where there is no airflow or respiratory effort for at least 10 seconds.
  • Central Hypopneas are defined by a 50 percent reduction in airflow and respiratory effort.
  • Mixed Apneas start just like Central Apneas. Instead of a total lack of respiratory effort for at least 10 seconds, towards the end of the apnea, there is an effort, which eventually breaks the event.
  • Obstructive Apneas sound scary. They are characterized by the lack of airflow for at least 10 seconds, accompanied by fruitless respiratory efforts.
  • Obstructive Hypopneas feature a 50 percent reduction in airflow, even as the respiratory effort is intact.
  • The RERA Count defines respiratory events not covered by the above definitions. Typically, a RERA is made up of a series of breaths, which feature increased effort and reduced airflow. Eventually, the event culminates in awakening.
  • In some cases, your sleep study breaks down respiratory events into categories defined by body position. Supine AHI, for instance, refers to your AHI while you are sleeping on your back.
  • Lateral AHI measures your AHI while sleeping on your side.
  • Prone AHI is your AHI while sleeping on your stomach.

In addition to respiratory events, there are other sleep-disruption triggers.

  • Limb movements also indicate disturbed sleep. Leg movements specifically are counted and expressed in your report through an index. This index consists of the number of leg movements per hour.
  • PLMS stands for Periodic Leg Movements of Sleep. This variable may show up on your report as well, as an event/hour index.
  • The PLMS Arousal index denotes the number of times periodic leg movements that have resulted in awakenings.

Oxygen saturation refers to the amount of oxygen in your blood during respiratory events and normal sleep stages. If your oxygen saturation falls below 95 percent, your brain is not getting enough oxygen. Severe sleep apnea sufferers may hit saturations as low as 60 percent during sleep.

Two variables are usually measured in this regard:

  • Mean SpO2, which represents the average oxygen saturation of your blood during sleep.
  • Lowest SpO2 during sleep.

Sleep times are the easiest to register and they complete the overall sleep picture nicely. Your sleep study will likely register Lights Out and Lights On times, Total Bed Time and Total Sleep Time. Based on this data, it then determines:

  • Your Sleep Efficiency, which is the number of minutes you spent sleeping, divided by the number of minutes you spent in bed.
  • Sleep Latency shows how quickly you fall asleep after lights out.
  • REM Latency measures the time it takes you to hit your first REM sleep stage.
  • WASO is a very important measure of sleep quality. It stands for Wake After Sleep Onset. It measures the amount of time you spent awake after you fell asleep and before lights on.

Sleep architecture (an analysis of your sleep stages) will likely have a section of its own in your sleep study. Sleep studies usually differentiate between four sleep stages: N1, N2, N3, and REM. They then specify the number of respiratory events (and even snoring) during the various stages.

    • N1 sleep is the lightest sleep stage, just after falling asleep. During this stage, it is extremely easy to awaken the sleeper. The N1 stage should take up about 3.4 percent of your total sleep time.
    • N2 sleep is also light, but it sees a reduction of body temperature and heart rate. It sets the stage for the deeper sleep stages that follow. N2 sleep should normally take up about 37.9 percent of your total sleep time.
    • N3 is the deep-sleep stage. It should make up about 22.4 percent of your total sleep time.
    • The REM sleep stage is where the majority of dreaming occurs. The first REM stage lasts for about 10 minutes. Each subsequent one is longer. The last one of the night is close to one hour in length. REM sleep should take up about 20-30 percent of your total sleep time.

The Arousal Index is set for every sleep stage. The Arousal Index normal range is the same as the AHI ranges described.

In addition to everything mentioned, your sleep study may include a special section for a CPAP titration study.

This study may take place on the same night as your “regular” sleep study. Its purpose is to fit you with a CPAP device after your sleep apnea has been positively diagnosed.

Based on the results of this study, your physician will recommend custom CPAP solutions for you.

Special Considerations when Interpreting Sleep Study Results

Your sleep study will allow your primary care provider to diagnose your sleep apnea (or to exclude it) and to recommend a course for treatment.

To properly interpret the results of the study, in addition to the data, your physician will also have to consider:

Sleep Consultations
    • The correlation between the results of the study and your sleep complaints.
    • Post-treatment residual problems and complications.
    • The effect of medications/supplements on sleep architecture.
    • The “first-night effect” if the study is performed in a laboratory. Due to unfamiliar surroundings and added stress, the test subject may find it difficult to sleep well.
    • The “reverse first-night effect,” when test subjects sleep better due to the unfamiliar location.
    • The fact that a negative sleep study may not rule out sleep apnea.


  1. Guy

    Dear Jeff,

    It’s understandable that you’re seeking clarity on whether REM Sleep Behavior Disorder (RBD) can be present even if a polysomnography (PSG) does not reveal sleep without atonia. Yes, it is possible for RBD to be present even if a PSG does not show sleep without atonia, particularly if there were no dream enactments during the night of the study. RBD symptoms can be intermittent, and a single night’s PSG might not capture the disorder’s manifestations.

    Your neurologist’s diagnosis based on your history and a second opinion confirming RBD after reviewing the raw data suggest that clinical judgment is also a key factor in diagnosing RBD. A repeat PSG might not always provide additional useful information, especially if your previous studies and clinical history already support the diagnosis. However, it can be interesting in tracking the progression or changes in the disorder over time.

    Since you’ve experienced side effects from clonazepam and melatonin, exploring alternative approaches seems reasonable. Focusing on lifestyle changes such as ensuring environmental safety, improving sleep hygiene, considering medicinal herbs, monitoring associated medical conditions and drug use, and working on developing more conscious sleep could be beneficial. These measures can often complement medical treatments and contribute to overall well-being and sleep quality.

    If you’re interested in discussing your specific case in more detail, feel free to contact me for a free consultation. As a sleep consultant, my approach would involve a comprehensive look at these aspects to tailor a plan suited to your individual needs and circumstances.

  2. Jeff Craig

    My question is if a polysomnography does not reveal sleep without atonia can a person still have REM sleep behavior disorder that does not show up on a polysomnograpy? My neurologist says that based on history I have the diagnosis. A second neurologist reviewed the raw data and agreed that I have REM sleep behavior disorder based on the review of the raw data. I did complete a year of treatment with clonazepam and melatonin. Could not continue on that course of treatment due to side effects. So, I went to Stanford for another opinion and I am now scheduled to have a repeat polysomnography but I am wondering if there is value in doing this repeat test if it too might not reveal REM sleep without atonia. I am not looking for medical advice…just trying to get an answer as to whether a person can actually have REM sleep behavior disorder and it not show up on a polysomnography simply because there were no dream enactments on the night of the polysomnography? Any answers or places to look for them would be greatly appreciated.

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