How to Understand your Sleep Study Report – Heather Holland


Heather Holland
Image by rawpixel from Pixabay

So you’ve had a sleep study, and you’ve been to the clinic for your results; you may have left the appointment feeling a little confused, and perhaps a little shocked.

Many patients are told they stop breathing and wake multiple times an hour. They have facts and figures thrown at them without much explanation and a new referral for CPAP Therapy.

Sleep apnea can be a confusing and scary diagnosis, and then having to navigate the even more baffling path of CPAP Therapy puts a lot of pressure on patients. Read: What is CPAP Therapy?

Here are 3 things to know about your sleep study report that will help you understand and take control of your treatment.


This is where I come in; most of my patients don’t have a clue how to read the double-page report. Once I go through the sleep study report outlining the following terms, everything is much clearer, and they leave their first appointment happier and more motivated for the road ahead.

Any CPAP therapist worth their salt should be explaining the sleep study report to you. It can empower you and give you a sense of control over your diagnosis. And I feel it can help in the road to successful CPAP use, and show you how much you can achieve during the CPAP trial — by comparing pre and post statistical data.

Let’s Dive In!

Hopefully, you’ve been given a copy of the sleep study report, if not, request one; a copy should be sent to your GP or referring physician as well.

AHI — Apnea Hypopnea Index

(may be indicated as RDI)

The first figure to look for on your sleep study report would be something called your AHI, which stands for your Apnea Hypopnea Index. It is the number of times you exhibited sleep apnea in an hour.

AHI = Number of Apneas & Hypopneas per hour

What are these obscure terms?

An apnea is a complete, or almost complete blockage of your airway (70% or more), whereas a hypopnea is a partial obstruction. Both result in a drop in blood oxygen, and an increase in carbon dioxide, which are the factors that play a role in waking you. The change in blood gas levels signals to your brain to wake you up so that you can resume normal breathing.

Both types of apnea above are factored into your result because both can result in an interruption to your sleep.

AHI determines how severe your sleep apnea is.

Here are a few stats to consider when looking at your AHI:

  • No/Minimal Sleep Apnea: AHI < 5 per hour
  • Mild Sleep Apnea: AHI ≥ 5, but < 15 per hour
  • Moderate Sleep Apnea: AHI ≥ 15, but < 30 per hour
  • Severe Sleep Apnea: AHI ≥ 30 per hour

With an AHI of 35, you have 35 apneas per hour that disrupt your sleep. You may not remember waking this many times, but when we conduct a sleep study, we see the brain wake 35 times. We see your brain wave activity (EEG) change from asleep, to awake with a burst of high-frequency activity.

The patient below only has hypopneas, and their AHI was 24. Each purple line represents a hypopnea, and the height of the line represents seconds. You can also see below that the hypopneas correlate and line up with the arousals (small awakenings).

But I’m not sleepy, so why treat my sleep apnea?

I have treated patients with mild sleep apnea who report extreme sleepiness,

but I have treated patients with severe sleep apnea who report no sleepiness.

If you have mild, moderate or severe sleep apnea, even if you do not have any subjective sleepiness, you should be treating your sleep apnea. It can have many more effects on your health than merely feeling sleepy.

Sleep apnea affects your cardiovascular health (Spicuzza et al., 2015) and every time you stop breathing, your heart is put under extreme pressure to re-stabilise your blood gases. It has been linked to heart failure, stroke, type II diabetes, anxiety & depression, to name a few (Morsy et al., 2019).

In addition, you may have your driver’s licence suspended unless you begin CPAP Therapy. This is especially so for commercial drivers licenses’.

Minimum Oxygen Saturation — E.g. Min Sa02

The next thing to look at on your sleep study is the minimum oxygen saturation; you will see this in the report as Minimum SaO2 or Saturation %.

When breathing normally, in a healthy adult, our oxygen levels will be above 95%. We should be breathing normally through the night also, so it should stay above 95%. However, if you have a look at your sleep study report, you may see a number far below this, such as 88%, or even 75%. This is how low your blood oxygen dropped while your airway was obstructed during sleep apnea.

The below saturation image is the same patient as above. The sharp drops to 70% are most likely the sensor losing contact with the finger. However, you can see small variations between 92% — 96%.

To be considered an apnea or hypopnea, there must be a 3%, or more, drop in blood oxygen. If you look carefully, you can see how the drop in oxygen correlates to the hypopneas.

If you have an AHI of 30, or only 7, your oxygen will drop each time you have an event of apnea. CPAP therapy works to reduce the number of times your blood oxygen drops, and how long it is reduced for. It’s designed to keep your blood gas levels balanced, thus keeping you asleep; reducing the number of times your sleep is interrupted and your body is in a stressed state.

The Hypnogram

The third item to look at, and possibly the most interesting, is the hypnogram; this is usually on the second page of the report. The hypnogram shows you what stage of sleep you are in, and how long for.

This nifty tool is a great way to explain the effect of sleep apnea on your sleep cycles and is really useful if you have a second sleep study while using CPAP equipment. You can compare your progress visually.

The hypnogram below is for the same patient we discussed previously.

W — awake, 1 — Stage 1, 2 — Stage 2, 3 — Stage 3 — Deep Sleep, R — REM (Rapid Eye Movement) the red line.

Ideally, you want to cycle through the sleep stages over approx 90 mins. And you want to cycle smoothly; you don’t want to be pulled out of the stages by sleep apnea which is evident with the patient below.

Below is what a hypnogram should look like during healthy sleep or the sleep of successful CPAP treatment. You can see a marked difference. The patient above is being pulled out of their sleep stage many times, and they have very little stage 1 or REM sleep.

Each sleep stage offers much in the way of brain and body recovery, for example, committing short term memory to long term memory — deep sleep and REM team up to deliver this advantage (Batterrink et al., 2018).

I really hope this article helps on your road to understanding your sleep apnea and successful CPAP treatment. Ask your CPAP Therapist to download a report from your machine, and compare your progress with your AHI and original sleep study report.

If you have any further questions about your sleep study, please speak directly with your sleep physician.

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