Your sleep study
What happens during a sleep study, step by step
Your doctor has ordered an overnight sleep study, known as polysomnography (PSG). You have been told what to bring and when to arrive. What nobody has really explained is what the night will be like, or why you will be covered in sensors.
This page walks you through the night in the order you will live it. It is written by a sleep technologist who runs these studies and trains the people who do them. Your sleep center's own instructions always come first.
The short version
- The study is completely painless. Nothing goes through your skin; everything sits on the surface.
- Getting hooked up takes about forty minutes, not ten.
- You will probably sleep worse than at home. The study is still usable in most cases.
- If it shows sleep apnea that needs CPAP, your machine still has to be set up, usually during a second night.
Before you come, and the real reasons why
Every leaflet gives you the same instructions. Hardly any of them tell you what they are for, even though almost all of them come down to one thing: getting a clean signal. An instruction you understand is an instruction you follow.
Clean hair, with no gel, hairspray or conditioner
The electrodes on your scalp need direct electrical contact with your skin. Any oily residue gets in the way and spoils the signal. That is the only reason for this rule, and it is the one people struggle with most when they have just been to the hairdresser.
No makeup on your face
The same goes for the electrodes near your eyes and under your chin. A little moisturizer or foundation is enough to make one come loose during the night.
No nail polish or artificial nails
The oxygen sensor works by shining light through your finger. Studies on nail polish disagree: the effect is small and mostly seen with dark colors. But the oxygen signal matters too much to take any chances, hence the rule.
A close shave, if you can
The electrodes under your chin do not stick well to a full beard, and they are already the ones most likely to come off when your jaw moves at night.
A comfortable top with a loose neckline
The belts go over a T-shirt, and the wires run under the collar. Anything tight around the neck makes the hookup harder and will bother you all night.
No alcohol on the evening of the study
Alcohol helps you fall asleep faster, but it delays and cuts down REM sleep, makes the second half of the night restless, and increases both the number of apneas and the drops in oxygen. The study would record a night that is not really yours, and that night is what your treatment would be based on.
A list of your medications, and your CPAP if you already have one
Some medications change the structure of your sleep, and the team needs to know about them to read your recording correctly. Do not change anything on your own: the goal is to record your usual sleep, not a special one-off night.
A word about your hair. "But I just had my hair done" is something we hear a lot, and it is a fair point. The honest answer: you will wake up with a little white paste in your hair, and it washes out with shampoo. This is a medical test, not a beauty contest, and the quality of the recording depends on it.
The paperwork detail that can send you home
You will rarely find it in patient guides, yet it is the one thing on this page that can cost you your night.
Having an appointment is not always enough. The room needs to be booked too.
At several centers, booking the sleep study and reserving the bed with the admissions office are two separate steps. Some patient leaflets spell it out: the room has to be booked with admissions before the day of the study. Other centers take care of it for you. There is no way to guess which kind yours is, so ask.
When you set the date, ask this exact question: is the room booked, or do I need to call admissions myself? Thirty seconds on the phone could save you several more weeks of waiting.
Check your arrival time, too. It varies from place to place: some units expect you as early as 5 p.m., others later, depending on how they schedule the hookup. Go by what your own center told you, not by what you read elsewhere, including here.
While we are at it
- Call ahead if you cannot make it. A sleep study ties up a room, the equipment and a member of staff for the whole night. Some centers charge a fee if you miss the study without letting them know. A call the day before frees the bed for someone on the waiting list.
- The bill usually comes later, by mail. You do not have to settle everything on the spot in the morning.
- Leave valuable jewelry at home. You will be sleeping in a hospital room and walking around a hospital unit at night.
- You will often be asked to fill out a questionnaire when you arrive, about how you usually sleep at home. It helps the team read your recording, because a night in the lab only makes sense next to what a normal night looks like for you. Answer honestly, including about alcohol, naps and irregular hours.
Arriving, and the evening
Every center organizes the evening differently, so this is the first thing to check with yours. At some, you are hooked up in the late afternoon and then have the evening to yourself. At others, the hookup happens later and you go to bed right afterward.
If you are hooked up early, you still have time to eat dinner, read or watch TV. Being wired up does not mean you are stuck in bed.
One rule covers the whole evening: sleep the way you usually do, just in a hospital. If you take a sleeping pill every night, no one will ask you to skip it. What we want to record is your real sleep, not an unusual night. Just let the team know what you take.
Illustration being redrawn
Horizontal timeline: arrival, hookup (40 min), free evening, biocalibration, lights out, recorded night, waking and unhooking, leaving.
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The hookup, sensor by sensor
This is the part that makes the biggest impression, by far. Once you are set up, you will be wearing about twenty sensors, with the wires gathered into a small box. A lot of patients take a selfie at this point to send to their family, which is a good sign: it means the nerves have settled.
None of it hurts. No sensor pricks or goes through your skin. Everything sits on the surface, held with conductive paste for the electrodes and tape for the rest.
Illustration being redrawn
Front-facing figure with four labeled areas: head and face (EEG, EOG, chin EMG, nasal cannula, snore sensor), chest and belly (ECG, two belts, position sensor), legs (movement sensors), finger (oximeter). A clean diagram, no photo.
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On your head and face
Scalp electrodes (EEG)
They record the electrical activity of your brain. This is the only way to know whether you are really asleep, and in which stage of sleep. Without them, we would be measuring your breathing, not your sleep.
Electrodes near your eyes (EOG)
Your eye works a bit like a tiny battery. Tracking its movements shows the moment you fall asleep and the stretches of REM sleep, when most dreaming happens.
Electrodes under your chin (EMG)
The muscles in your chin go slack during REM sleep. That contrast is what tells REM sleep apart from light sleep.
Thermal sensor and nasal cannula
Two ways of measuring your breath. The thermal sensor picks up the difference in temperature between the air you breathe in and out, which is enough to catch pauses in breathing. The pressure cannula is more sensitive and catches partial drops in airflow, called hypopneas.
Snore sensor
A small sensor placed in the hollow at the base of your throat, which turns vibrations into a signal. It measures your snoring instead of relying on your partner's memory.
On your chest and belly
Two elastic belts
They measure how your chest and belly move. This is what separates obstructive sleep apnea, where your body keeps straining to breathe, from central sleep apnea, where the signal to breathe stops. The difference changes the treatment.
Two heart electrodes (ECG)
They follow your heart rhythm through the night and can reveal rhythm problems that are sometimes linked to apneas.
Position sensor
It tells whether you are on your back, your front, either side, or sitting up. Some people have apneas mainly on their back, and knowing this can point toward positional therapy.
On your legs and finger
Sensors on both legs
They record your leg movements during sleep. These movements do not always involve both legs at once, which is why the Belgian NIHDI (INAMI/RIZIV) convention requires recording both.
Oxygen sensor on your finger
It measures your blood oxygen saturation. It shows the drops in oxygen that follow pauses in breathing, and it is part of how hypopneas are defined.
The order matters more than you might think
A well-run hookup follows a set order. For example: heart electrodes and belts first, then the legs, then your face, then your scalp, and finally the oxygen sensor. There is a reason for this. It means you do not have to move around once the head electrodes are on, and each signal can be checked as it goes.
The nasal cannula comes last because it is the only truly annoying part: two small prongs at the opening of your nostrils that tickle for a few minutes and then fade into the background. Putting it on at the very end spares you twenty minutes of needless discomfort.
Why we scrub your skin before the electrodes go on
Before each electrode goes on, the skin is cleaned and lightly scrubbed with a slightly gritty paste. People are often surprised by this, but it is key. An electrode picks up tiny electrical signals, measured in millionths of a volt. The outer layer of your skin, with its natural oils, gets in the way of that signal. Prepping the skin lowers that resistance so the recording comes through clearly. It is also why you are asked to arrive with nothing in your hair.
Biocalibration: when it all starts to make sense
Once all the sensors are on, you will be asked to do a series of things that may seem odd: move your eyes, clench your teeth, breathe through your mouth and then your nose, count out loud, move one foot and then the other.
This routine is called biocalibration, and each step tests a specific sensor. If your team does it, that is a good sign: they are checking that everything works before the night starts, rather than finding out in the morning.
Ask to see the screen
There is a screen in the room that shows your signals live. There is nothing to stop you from asking to see it during biocalibration, and it is the best way to understand what is going on.
Close your eyes and slower waves show up on the screen. Open them and the waves disappear. Clench your teeth and one line starts jumping. Take your finger out of the sensor and the oxygen line drops. Move one foot and only one leg responds.
The belts are the most striking part. First you breathe normally, and the two lines, chest and belly, rise and fall together. Then you are asked to move them against each other, pushing your belly out while your chest sinks in. The two lines now go in opposite directions. That is exactly what your body does during an obstructive apnea, when it fights against a closed throat. With that one test, you have seen what the study is looking for.
Illustration being redrawn
Two panels side by side. Left, normal breathing: chest and belly lines in sync. Right, effort against a closed throat: the lines move in opposite directions. Simple labels, no jargon.
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Illustration being redrawn
Sleep lab bedroom: a wired-up patient sitting on the edge of the bed while a technologist turns the screen toward them. Calm and reassuring, not clinical or frightening.
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The night
The camera
In most sleep labs, an infrared camera records your bed all night. You will probably spot it as you walk in, and it is better to know in advance than to discover it as you lie down. It serves a real purpose: it lets the team match an unusual signal with what your body was doing at that moment, which matters most for leg movements and unusual behavior during sleep. There is no camera for a home study.
Someone is watching over you
In the lab, a nurse or technologist watches your signals live all night; the NIHDI convention requires it for every PSG. If an important sensor comes loose, someone will come in and fix it, which may wake you briefly. It is annoying, and some patients find it hard. But it is the right call: a two-minute wake-up beats losing the recording for the second half of the night.
Eight hours, but not eight hours of sleep
The recording starts when you are ready to sleep and the lights go out, and it runs for at least eight hours. Nobody expects you to sleep for eight hours. That time is simply there to give you a chance at a normal night.
Getting up, moving around, going to the bathroom
You can still go to the bathroom; just let the team know and they will unplug what is needed. Otherwise, try not to get up without a reason while the recording is running. Sleeping on your side, turning over, moving around: all of that is normal and expected. The sensors are attached with that in mind, and your position is one of the things being measured.
"I slept terribly, it was all for nothing"
This is what we hear every morning, and the recording usually proves it wrong.
Feeling like you were awake all night is far more common than actually being awake all night. When the recording is analyzed, it usually shows the person did sleep, sometimes more than they thought, and the study can be used.
In the rare cases where the recording really cannot be read, the study is repeated. That is not a failure, and it is not your fault.
The morning
Unhooking is quick, a few minutes at most. The question everyone asks is about the paste left in their hair. It washes out with ordinary shampoo. If your center has no shower or time is short, you may leave with a few white streaks, which is nothing to worry about.
If you did the study at home, the instructions are different and often misunderstood: take off the recorder, but leave all the sensors plugged into it. Just peel them off your body, pack everything up and bring it back as it is.
Results are not instant. One night produces hours of signals that a trained member of the team reviews by hand, and the doctor then checks. Your results are explained to you at a follow-up appointment and sent to your GP.
The second night nobody mentioned
If the study shows sleep apnea that needs CPAP, the right pressure still has to be found. Most of the time, that means a second night in the sleep lab. The news is rarely welcome, especially right after the first night.
This night is called a titration study, and it does two things.
It sets up your machine
Your starting CPAP pressure is often estimated with a formula based on your apnea index, neck size and body build. That is a best guess. During the titration night, the pressure is adjusted over the hours to find the one that actually works for you.
It completes your file
The NIHDI convention requires the pressure to be set, and shown to work, before treatment starts. The application for reimbursement can only be sent after this step. The first night makes the diagnosis; the second confirms the treatment.
Not always in the hospital. The convention does not allow the diagnosis and the titration to be done on the same night. However, for some patients (severe apnea, without certain other health conditions), the doctor may choose to do the titration at home with an auto-CPAP machine, then check it with a respiratory polygraphy. That decision is up to your doctor.
Two comparisons help explain why this night is more than a formality.
It is like the follow-up X-ray after surgery: it checks that everything was put in place correctly.
It is also like the blood test after a few weeks on a new medication: it checks that the dose is right.
In practice, the setup is the same, with one exception: no nasal cannula, since you will be wearing the mask. Otherwise, you get the same electrodes and belts.
One tip makes a big difference: get used to your CPAP at home before the second night, if your center has already given you one. A night in the hospital is uncomfortable enough. Arriving already used to the mask means you are not dealing with two new things at once.
In the hospital or at home
Both options exist in Belgium, and the choice has consequences worth knowing about before you decide.
Illustration being redrawn
Two-column comparison: sleep lab (what is measured, reimbursement, wait time, second night) versus home (fewer measurements, paid out of pocket, short wait, machine bought outright).
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| In the sleep lab | At home | |
|---|---|---|
| What is measured | Your sleep itself, through the head electrodes, as well as your breathing | Usually your breathing only (respiratory polygraphy), with no direct measure of sleep |
| Wait time | Often long, since beds are limited | Usually shorter |
| Cost | Reimbursed by public health insurance; you may still pay a share | Paid out of pocket, often several hundred euros depending on the provider |
| CPAP reimbursement | Opens the door to the NIHDI convention, if you meet the criteria | Does not qualify: the convention requires polysomnography at an accredited sleep center |
| Second night | Usually, for titration | No second night; you buy the machine directly |
A home test has real advantages: it is quick, you sleep in your own bed, and you skip the second night. Its main drawback is not technical but financial. Outside the convention, you pay for the machine and its follow-up yourself.
On the other hand, a machine bought outside the convention comes with no minimum-use requirement. You will not have to show an average of four hours of use a night.
One point applies to both: a breathing-only test, with no measure of sleep, counts events per hour spent in bed, not per hour actually asleep. Since nobody sleeps the whole time they are in bed, this tends to underestimate how severe the apnea is. That is why the American Academy of Sleep Medicine recommends polysomnography when a home test comes back negative or inconclusive.
Frequently asked questions
Next step
Your study was positive. What now?
The first few weeks with CPAP often shape how things go from there. Here is what to expect, week by week, and how to solve the most common early problems.
Sources
- NIHDI (INAMI/RIZIV), Convention on the diagnosis and treatment of sleep apnea syndrome, in force since January 1, 2018 (articles 4, 6, 14 and 17, annexes 2 and 3; official French text)
- Elbaz M., Léger D. et al., Le guide des Techniciens du sommeil: aspects pratiques et diagnostic des troubles du sommeil, French Society of Sleep Technologists (SFTS) and Philips Respironics (in French)
- Berry RB, Budhiraja R, Gottlieb DJ et al. Rules for Scoring Respiratory Events in Sleep: Update of the 2007 AASM Manual. J Clin Sleep Med. 2012;8(5):597-619.
- Kapur VK, Auckley DH, Chowdhuri S et al. Clinical Practice Guideline for Diagnostic Testing for Adult Obstructive Sleep Apnea: An AASM Clinical Practice Guideline. J Clin Sleep Med. 2017;13(3):479-504.
- Ebrahim IO, Shapiro CM, Williams AJ, Fenwick PB. Alcohol and sleep I: effects on normal sleep. Alcohol Clin Exp Res. 2013;37(4):539-49.
- Kolla BP, Foroughi M, Saeidifard F et al. The impact of alcohol on breathing parameters during sleep: a systematic review and meta-analysis. Sleep Med Rev. 2018;42:59-67.
- Hinkelbein J, Genzwuerker HV, Sogl R, Fiedler F. Effect of nail polish on oxygen saturation determined by pulse oximetry in critically ill patients. Resuscitation. 2007;72(1):82-91.
- Miljeteig H, Hoffstein V. Determinants of continuous positive airway pressure level for treatment of obstructive sleep apnea. Am Rev Respir Dis. 1993;147(6 Pt 1):1526-30.
