How Does a Sleep Study Work?

A sleep study provides detailed information about the unique physiology of sleep which is instrumental in the evaluation of sleep and the diagnosis of sleep disorders.

During the sleep study, electrodes and sensors are non-invasively attached for recording the electrical activity of the brain and muscles as well as for recording breathing, oxygen levels, and heart rate during sleep.

The patient concerns about why a sleep study is needed, what happens if you have trouble sleeping during the study and safety of the procedures are discussed in the article.

The goal of the sleep evaluation is for an accurate diagnosis and the determination of effective treatments for a sleep problem. (1)  Given the current conditions of COVID-19 many sleep centers are not open.  In this case a home sleep test is the only option.

Sleep Study:  What is it and How Can it Help?

If you have trouble with your sleep or you feel drowsy during the day even after a full night’s sleep, you are not alone.  Several large surveys in recent years have shown that up to one third of the population has significant difficulty with sleep.  Sleep disorders can affect everyone from babies to the elderly.

But why do we care about poor quality sleep or too little sleep?  There is a common misconception that missing a few hours of sleep is really not all that important, and at worst it is a minor inconvenience.  However, poor sleep and daytime drowsiness have been linked, among other problems, to increased numbers of automobile accidents, poor work and school, performance, diabetes, heart disease, high blood pressure, and depression.  Thus, poor quality sleep and too little sleep make us vulnerable to a decrease in our quality of waking life.  If your sleep has begun to interfere with your health and quality of life, your doctor may refer you to a Sleep Center for an overnight study in order to discover the cause of your symptoms. The purpose of this article is to give you a description of what occurs during these studies.

You may see the terms polysomnographic evaluation or nocturnal polysomnogram used in referring to an overnight sleep study.  In a nutshell, a polysomnographic evaluation consists of recordings of your brain and bodily activity during a study sleep which is performed overnight in a Sleep Center. (2)  Polysomnographic evaluations provide detailed, invaluable information about the activity of the body and brain during sleep that cannot be discovered during waking as the result of major physiological changes during sleep.  These changes can be associated with disorders that are unique to sleep. We all know how sleepy, drowsy, and out of sorts we feel during the day if we do not get a good night’s sleep.  So really the purpose for sleep is not just to sleep at night, but to ensure our overall health and optimal functioning during the day.

EEG Stages of Sleep from a sleep study

Full polysomnographic monitoring, which we discuss in detail later, is the “gold standard” for the diagnosis and treatment of sleep disorders. (3) Recently, new technology has been developed to allow limited polysomnography to be performed with portable monitoring devices which are worn at home. (4) There are advantages and disadvantages to both types of monitoring, and the details of portable monitoring studies are discussed in a separate article. (5)

Before we discuss the details of what happens during a sleep study, let’s address some very general questions that you may immediately have when you are first referred to a Sleep Center.

 Why do I need a sleep study?

 If your doctor suggests that you prepare for a sleep study, your initial reaction may be “Why do I even need a sleep study?  I’m a little sleepy during the day, but I don’t have any trouble sleeping at night.  My sleep is fine!”

After you ask this question, remember that when you are asleep, you are the most unreliable of observers as to what your body is doing and to your behavior during your sleep.  It is very obvious, but the reason you are not a good witness is that you are asleep!  For example, patients commonly deny that they snore loudly.  Sometimes this snoring is so loud that it wakes up every person, and even pets, in the household.  However, the sleeper remains blissfully unaware of these loud, disruptive sounds.  It seems even more unbelievable that your bed partner might tell you that, besides loud snoring, you stop breathing during sleep over and over throughout the night.  Sometimes these periods last only a few seconds, but often they last 30 to 60 seconds or even louger.  You are probably thinking, how is it possible to snore so loudly and to stop breathing during sleep without even being aware of this?  It is precisely because you ARE asleep that you do not have direct knowledge of your bed partner’s observations.  This is why the overnight polysomnographic evaluation becomes an essential tool for discovering precisely what is occurring in your body during sleep.

What happens if I can’t sleep during my sleep study?

Interrupted sleep impacts mood more than lack of sleep, study finds

Another immediate concern that you may have is whether you will be able to sleep during your study.

The thought of spending the night in a new environment with a multitude of recording electrodes and sensors attached to your head and body while you are being observed and heard on a closed circuit monitoring system throughout the night is enough to cause distress and anxiety among even the bravest person.  The circumstances surrounding a sleep evaluation are so remarkably different from our usual sleeping experience at home that it may seem as though no one could possibly sleep normally under these conditions.

It is common to have some difficulty sleeping in a new and unfamiliar environment.  Almost everyone has had the experience of staying in a hotel room and then having some difficulty falling asleep.  In fact, there is a phenomenon called the “first night effect” which has been studied in persons sleeping for the first time in new environments. However, some minor difficulty falling asleep in a new place due to nervousness or anxiety is normal and does not mean that the results of the sleep study will be invalid.  Not having your completely usual sleep such as you have at home still allows, in most cases, for a sleep disorder, if present, to be recognized and treated. Sometimes, the sleep doctor will order a home sleep study to evaluate for sleep apnea.

One important note, however, is that if you are having difficulty falling asleep because the recording devices are causing you discomfort, be sure to call the technician so the monitoring devices can be adjusted.  This does not mean that the sensors can be removed since they all record important information for diagnosing your sleep problem.  However, the technician can typically make adjustments for greater comfort so that you can sleep.

You may be surprised at how well you sleep under the conditions of the sleep study.  The majority of patients usually do not have as much difficulty sleeping as they anticipate, and almost everyone sleeps enough during their study to make an accurate diagnosis.  In fact, it is not unusual for patients to report that the quality of their sleep is even better during their study than it is at home.  If your sleep problem has become debilitating, you may feel relieved that the source of your sleep problem is finally being directly examined with specialized testing, thus decreasing your concern about not sleeping.  Sleeping medications are typically not provided by the technicians on the night of study, but in some circumstances the use of medications will be discussed with you by the sleep specialist prior to your study.

Most patients spend one or sometimes two nights in the Sleep Center and in some cases a series of daytime nap tests called the Multiple Sleep Latency Test, referred to as the MSLT, may be performed during the day following the overnight study.  In addition, depending upon the results a “split night” study may be performed for further evaluation of sleep apnea (stopping breathing during sleep).  However, this will be discussed with you in advance of your study.

Probably the single most important factor in allowing yourself to fall asleep during your evaluation is to keep a positive mindset and focus on the benefits that you will obtain as the result of the information which is being collected.  Remember that the results will provide you with detailed and important information about the cause of your trouble with sleep which will, in turn, lead to potentially effective treatment.  If you feel as though you are having trouble falling asleep, try thinking about these positive benefits and relax your body with deep breathing and muscle relaxation to encourage sleep.

Finally, remember that the technical staff can hear and see you at all times during the night should you have any difficulty or concerns.  In most Centers, all you have to do is speak and the technician will enter the bedroom to help you.

What happens during a sleep study?

Now that we have addressed these initial concerns, let’s discuss what happens before your study, during your study and after your study.  Of note is that there may be some variation in these procedures depending upon the Sleep Center.  Always check with the Sleep Center where you are scheduled if you have questions.

Before your sleep study . . .

 You will be asked to make some preparations before you report for your evaluation so that the most accurate information about your sleep can be obtained.  Again, check with the Center where you are scheduled if you have questions..

  • On the days and nights preceding your study, keep your usual schedule of going to bed and getting up. This is important so that you do not alter your body’s usual sleep-wake rhythms.  In the case of shift workers sleeping during the day and working at night, you will most likely be asked to keep your usual schedule and the sleep study may be performed during the day..
  • Caffeine containing beverages such as coffee, tea, and cola drinks or chocolate, which also contains caffeine, may disrupt your sleep. You should not consume caffeine beginning in the afternoon prior to the night of your study.  Heavy or spicy meals can also disturb your sleep, so eat a light supper before reporting for your study.
  • There is a myth that alcohol improves sleep by allowing you to fall asleep more quickly. Studies have shown that although alcohol can, in fact, cause you to fall asleep more quickly, it causes fragmented, interrupted, restless sleep.  Alcohol can also alter your sleep stages and breathing.  You should not drink alcohol on the day of your study.
  • It is important to have clean hair and skin so that the recording sensors can be attached for clear and accurate signals. Taking a shower and washing your hair before the study will allow better placement of the recording devices.  Also, do not use using lotions on your skin or hair products on the night of your study since these can also interfere with good quality recordings.
  • A wig or hair weave can interfere with the accurate placement of sensors on the head, and you should remove these before reporting for your study. If you do not have much hair on your head, bald truly is beautiful for the purpose of applying recording sensors.
  • Bring your pajamas and a robe with you. As the recording sensors are being applied, you will probably be most comfortable in two piece sleeping wear.  If you have a favorite pillow or blanket, check with the Sleep Center to see if you can use these during the study.
  • If you take medications before bed, discuss the use of these medications with the sleep specialist prior to your study, and bring these medications with you if necessary. If you need to eat a snack near bedtime, also bring this with you, again discussing this with your sleep specialist.
  • A sensor consisting of a comfortable, padded clip will be placed on the tip of your index finger for measuring your oxygen levels throughout the night. This sensor works by passing a light beam from one side of the sensor through your finger tip to the other side of the sensor.  Artificial nails or heavy nail polish may interfere with accurate readings of your oxygen levels.
  • Before going to bed you will probably be asked to complete a brief questionnaire about your activities that day. If you wear glasses for reading, be sure to bring your glasses so that you can complete this questionnaire.
  • Sometimes patients ask if a bed partner can sleep in bed next to them during the overnight evaluation. Having another person in the bed can be very disruptive to the study, and in general another person typically cannot stay in the same room or bed as the person having a study performed.  There may be a cot or another bedroom available if another person stays with the patient.  However, in some circumstances, such as with a child’s study or a patient requiring special care during the night a caretaker may be allowed to sleep in the same room, but not on the same mattress.  Be sure to check with your Sleep Center if this issue arises.
  • Cell phones are an important part of our lives, but be prepared to turn your cell phone or DVD player off once the study has begun. Light and sound have the potential to disturb your study.  Remember that your most important activity this night is to sleep!

During your study . . .

 You will be asked to report to the Sleep Center two hours or so prior to your bedtime in order to prepare the equipment and attach the recording sensors and equipment to your body.  It is important to report at your assigned time so that there is enough time to prepare you for the night and so that you spend at least six hours in bed.  Very short times in bed may not allow for enough sleep to determine a diagnosis.

You will sleep in a private bedroom where the study is performed.  All of the actual monitoring equipment is in a separate room.  After you have changed into your pajamas, a technician will come into your room and begin applying the electrodes and sensors for your recording.

Is it painful to have these electrodes and sensors applied?  The answer is no.  The technician attaches each of the electrodes and sensors to the surface of your skin and to your scalp with cotton gauze and/or surgical tape after cleaning.  If you experience any irritation as the technician cleans your scalp and skin, be sure to speak up.  None of the recording devices break the skin, and none are placed beneath the skin.  If you have a known allergy to tapes, be sure to let the technician know in advance.

What is monitored during a sleep study?

Although there can be variations in the types of sensors which are used, during most sleep evaluations the following recordings will be made.

  • Brain activity (electroencephalogram or EEG). The technician will measure your head and apply electrodes to your scalp using a universal system of measurement for recording from several different parts of your brain.  Your scalp will be cleaned and the sensors will be filled with a small amount of electrode jelly for keeping good contact with the scalp.  A small gauze pad will be applied over each electrode to hold it in place.
  • Eye movements (electro-oculogram or EOG). Eye movements occur not only during waking but they also occur during rapid eye movement sleep (REM sleep) which is the dreaming stage of sleep.  Sensors will be placed near both of your eyes for recording these movements in order to detect REM sleep.  These electrodes may be attached with surgical tape or small gauze pads.
  • Muscle activity (electromyogram or EMG). Activity of the chin muscles is typically recorded from sensors placed on top and underneath your chin.  Another special feature of REM sleep or dreaming sleep, besides the presence of eye movements, is that the muscles of the chin and other skeletal muscles in the body (with the exception of the muscles controlling eye movements) lose their tone during this sleep stage.  For this reason we are unable to move voluntarily during this sleep stage.    These recordings help determine when REM sleep occurs.  In addition, the EMG is recorded from muscles in the leg, the anterior tibialis muscles.  This muscle activity tells us if periodic limb movements (often abbreviated as PLMS) are present.  PLMS are rhythmic twitching of the leg muscles during sleep which can be very disruptive to sleep, but which are most often unknown to the sleeper.  In addition, depending upon your symptoms, muscle activity may be recorded from the arms to evaluate twitching or from muscles near the jaw to evaluate tooth grinding, a condition called bruxism.
  • There are several different sensors which are used to record your breathing during the study.  In order to record the pressure of air movement through your nose, small tubes similar to an oxygen cannula will be placed just inside both your nostrils.  A small sensor will also be placed near your nose for measuring the temperature of air moving in and out of your nose.  An elastic band will be placed on your upper chest and also on your abdomen for measuring movement of your diaphragm during breathing.  Finally, a small sensor may be taped on your chest which senses your body position, that is whether you are sleeping on your back, sides, or stomach.  All of these sensors allow us to determine if you have episodes of stopping breathing during sleep, in what body position they occur, and what type of sleep apnea is present.
  • Oxygen saturation (SpO2) A small padded clip will be placed on your index finger to measure oxygen levels in your blood.  This test is often referred to as oximetry or nocturnal oximetry.  With this device a light beam passes from one side of the clip, through your finger, and to the other side of the clip.  Nocturnal oximetry allows us to determine if there are significant decreases in your oxygen levels during sleep, particularly if you stop breathing.
  • Heart rate (the electrocardiogram or ECG). Changes in heart rate and irregularities in heart rhythms can occur in different sleep stages and with some sleep disorders, most prominently sleep apnea.  One or two electrodes will be taped to your chest for recording the activity of your heart.

Some patients, in addition to these sensors, will also have a mask which covers the nose or the nose and mouth, depending upon the circumstance, for further evaluation of sleep apnea.  This mask carries pressurized air from a machine and through the mask in order to keep the throat open and breathing regular.  This device is a continuous positive airway pressure (CPAP) machine.  Your sleep specialist will discuss whether or not CPAP will be used during your evaluation with you before the study.

“There is so much equipment attached to me!  Is it safe?  One concern that you may have is whether the recording sensors and equipment are safe.  Nothing is passing into your body from the recording electrodes, sensors, or any other equipment.  The equipment is safe.  There are electrical safety checks and equipment inspections routinely performed in the Center to ensure patient safety.  The tiny electrical potentials generated by your brain and muscles are being magnified and passed through your electrodes into the monitoring room recording devices.

After the sensors are applied, a process which typically takes about an hour, you will probably be allowed to read or relax until just prior to the time that you usually go to bed.

All of the equipment which is used to record the activity of your body during sleep is housed in a monitoring room separate from your bedroom.  Your bedroom should be quiet.  No radios or television will be left on during the study.  This is so that the technician can hear you clearly, and so that your sleep is not disturbed.  There is a closed circuit television system which will allow the technician to see you and talk to you from the monitoring room.  The technician will be continuously observing you and monitoring your recording throughout the night.  Also be aware that the technicians are trained in cardiopulmonary resuscitation (CPR) and the Sleep Center has procedures for implementing emergency medical procedures.

Starting the sleep study.  When your bedtime arrives, the technician will ask you to lie down in bed, and your recording devices will be connected to a specialized recording box which is in turn attached to a cable going into the recording equipment in the monitoring room.  To start the sleep study the technician will talk to you over the two-way intercom system.  You will be asked to perform a series of maneuvers in order to test the electrodes.  For instance, some of these instructions typically include opening and closing your eyes, moving your eyes, sticking out your tongue, flexing your legs, and holding your breath.  By following the instructions, the technician can make adjustments to the recording and to be sure that all of the sensors are operating correctly.  You will then be asked to try and fall asleep.  At this point the study has begun.

Getting up during the night.  If you need anything during the night, you can speak and the technician will hear you in the monitoring room.  Many persons get up during the night to use the restroom.  It is a simple procedure for the technician to get you up, and the electrodes are not removed when this occurs. If you need to use the restroom or need anything else, call the technician and your electrodes will be “unplugged” from the recording cable.  You should not try to detach yourself and get up without notifying the technician.

After your study…

Before your study begins, the technician will ask you at what time you want to get up in the morning.  You will usually be scheduled to spend at least six hours in bed during the sleep test.  After you are woken up and out of bed, the technician will remove the recording sensors, and you will be asked to complete a brief questionnaire about how you slept during the study.  You will then be free to leave the Center.

The recordings of your sleep will be scored for sleep stages as well as for any breathing abnormalities or movement events.  Standardized rules published by the American Academy of Sleep Medicine are used for scoring sleep studies.  After scoring, the records, along with your sleep and medical histories, will be reviewed by a board certified sleep specialist, and a report which contains the results and treatment recommendations will be generated.  The report will be sent to your referring doctor who may give you these results or you may be scheduled to return to the Sleep Center for discussion with the sleep specialist.

Where should I go for a sleep study?

Sleep Centers which are accredited by the American Academy of Sleep Medicine (AASM) have passed rigorous safety and technical standards in order to perform high quality sleep studies.  Sleep specialists who have passed examinations leading to board certification in sleep medicine are affiliated with these Centers and are qualified to interpret the sleep study as well as to provide treatment recommendations.  In addition, technicians employed by these Centers are typically board certified by the Board of Registered Polysomnographic Technologists (BRPT).  This accreditation ensures that the technicians adhere to high standards for patient monitoring and that they are certified in emergency cardiopulmonary resuscitation (CPR) procedures.  Just as you would choose a board certified physician for your usual medical care, choosing an AASM accredited Sleep Center for your sleep care will ensure that high quality standards are being used to diagnose your sleep disorder.    For a list of board certified sleep physicians, visit the website of the American Board of Sleep Medicine.

You can see from this review that a polysomnographic sleep study evaluation is a detailed, comprehensive process.  Although it may seem complex, it is not a difficult study for you, the patient, to have performed.  All you have to do is lie back, relax, and sleep.  What other medical test have you had in which the primary requirement was to sleep, and how much easier could a test be?  Anticipate having a great night during your evaluation!

Further readings on sleep studies

Polysomnography by Carmel Armon,

No more sleepless nights (revised edition) (book)

Peter Hauri, Ph.D. and Shirley Linde, Ph.D.

John Wiley and Sons, 1996

The mystery of sleep:  Why a good night’s rest is vital to a better, healthier life  (book)

Meir Kryger, M.D.

Yale University Press, 2017.

Author: Kristyna M. Hartse, Ph.D.

Diplomate, American Board of Sleep Medicine (ABSM)

Fellow, American Academy of Sleep Medicine (AASM)

Registered Polysomnographic Sleep Technologist (RPSGT)

Registered Sleep Technologist (RST)

Certification in Clinical Sleep Health (CCSH)

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6 thoughts on “How Does a Sleep Study Work?

  1. Joe M Reply

    Reply to Rob (I can’t get it to work on the website).

    That’s my experience with a Zeos machine. It said I had an 87 out of 100 sleep and it was the worst night’s sleep I’d ever had. If their measure doesn’t match our experience then what value does it have? And if 99% of people need your cure, then maybe you’re testing incorrectly.

  2. Joe M Reply

    What about people that require stomach sleeping? I always see back sleeping in a hospital, and have NEVER been able to achieve that (thankfully no days spent in a hospital yet)

  3. Nelson Fogerty Reply

    It’s interesting that you mention that getting a sleep study done can tell you if you have sleep apnea. I’ve been experiencing a lot of drowsiness during the day, so I’m thinking about going to a doctor for a test to see if I have sleep apnea. I’m going to look for a reputable sleep apnea clinic in the area that I can use.

  4. Alan Drake Reply

    With the Home Sleep Study what is the percentage of people that fail? ie need a system at night to sleep. My question seems to be shared by many, the conclusion is these home tests are just a money makers.

  5. Rob Reply

    I just had a sleep study done. And out the almost 9 hours of being in bed for my actual study, I only slept 30-60 minutes towards the end of the study..(9:30pm start and 5:45am wake up time. What bothers me is that I did not sleep 100% of the time for the first part of the test, just laid there eyes open tossing and turning, then the tech comes in at 2am and puts the cpap mask on me and says good night.
    I laid there for another 3.5 hrs with the mask on and then in the morning was told “you dont like this mask do you”. Then he told me I needed to have the cpap in my life. He also told me that 99% of the people that go there get a machine. By far the worst night sleep of my life…

    I sleep great at home and have tons of energy even after work to do work on projects till late, only reason I went is because my girlfriend is a SUPER light sleeper and says I snore alot and “maybe” stop breathing.. How can this Technician determine that I need this machine for the rest of my life if i didn’t even fall asleep during the first test to even hear if i snore? Lol. I just feel like this could be a money thing, it felt very unprofessional. And as a ex- auto technician its seems like I would want to hear the “engine noise” while the engine was actually running before making the conclusion that “you need a new engine $$”….

  6. Daphne Gilpin Reply

    Thanks for explaining that the technician will start by asking what time we want to wake up in the morning. I’ve been having trouble sleeping lately and have been considering using a sleep center. Thanks for giving me an idea of what to expect!

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