Overnight polysomnography
Table of contents
Overnight polysomnography, also known as polysomnography or a sleep study, enables different bodily functions to be monitored simultaneously whilst a person is asleep. The recording helps to understand how sleep is structured and to link awakenings, breathing, changes in oxygen levels or certain movements to the exact time of night when they occur.
What is a night-time polysomnography and what does it enable us to study?
A nocturnal polysomnography is a test that simultaneously records various physiological signals whilst a person is asleep.
Unlike a recording that focuses solely on breathing, it enables us to identify when the person is actually asleep, recognise the different stages of sleep and observe what is happening at those times in terms of breathing, oxygen levels, muscle activity and certain movements.
To obtain this information, small sensors are placed on different parts of the body. Depending on the reason for the study, the recording may be supplemented with synchronised video.
The aim is not to analyse each signal separately, but to reconstruct how the night unfolds and to establish links between different changes. For example, it is possible to determine whether a respiratory disturbance occurs during a specific sleep stage, whether it is accompanied by a drop in oxygen levels, or whether it causes the person to wake up.
The criteria used to analyse a polysomnography are standardised and include the assessment of sleep stages, awakenings, respiratory events, movements and certain cardiac events.
As well as monitoring breathing and oxygen levels, the test reveals when the person is asleep and which stage of sleep they are in. This makes it possible to link apnoeas, awakenings, movements and other changes to the specific moment at which they occur.
Would you like more information about overnight polysomnography?
The team can provide information on the preparation, how the evening will unfold, and the situations in which this study may provide useful insights.
What signals are recorded during a sleep study?
The sensors are arranged to answer three main questions: how you are sleeping, how you are breathing, and what other changes occur during the night.
How sleep is detected
Small electrodes placed on the scalp record electrical brain activity. Together with sensors situated near the eyes and on certain muscles, they enable us to distinguish between wakefulness and the different stages of sleep.
Among these stages is REM sleep, a phase characterised, amongst other things, by rapid eye movements and specific changes in muscle activity.
This information reveals how long a person has actually slept and how their sleep was organised throughout the night.
How breathing is studied
Sensors near the nose and mouth record airflow. At the same time, straps placed around the chest and abdomen provide information on the effort the person is making to breathe.
A pulse oximeter, usually placed on a finger, measures the oxygen concentration in the blood and allows changes in this level to be monitored throughout the night.
What other changes can be recorded
The study may include heart rate, leg movements, body position and snoring. When investigating specific nocturnal movements or behaviours, synchronised video may be added to link observations to sleep stages and the other recorded signals.
When might a night-time polysomnography be recommended?
The test may be requested when it is necessary to observe directly what happens during sleep and the patient’s medical history or a more limited investigation does not provide sufficient information.
Respiratory disorders during sleep
One of its most common uses is the investigation of sleep apnoea and other disorders in which breathing may be disrupted during the night.
Polysomnography makes it possible to distinguish between different types of respiratory events, to check whether they occur during specific sleep stages or in certain sleeping positions, and to assess whether they are accompanied by awakenings or changes in oxygen levels.
Movements during sleep
It can be used to investigate repetitive limb movements and determine whether they fragment sleep or are associated with awakenings.
Nocturnal behaviours or episodes
Some people exhibit movements, vocalisations or behaviours during the night that require a more detailed assessment.
In certain parasomnias – a term encompassing abnormal behaviours or experiences that occur during sleep – the recording of physiological signals and, where appropriate, video footage can help to characterise the episodes.
Excessive daytime sleepiness
In people with severe daytime sleepiness, polysomnography may form part of a wider assessment. In certain cases, it is carried out prior to specific tests designed to assess how easily a person falls asleep during the day.
Polysomnography is not part of the routine investigation for all cases of insomnia. It may be considered when there are additional symptoms suggesting another sleep disorder that requires a night-time recording.
The service at Memorial addresses chronic insomnia, excessive daytime sleepiness, hypersomnias, narcolepsy, sleep-wake rhythm disorders, periodic limb movements and parasomnias, amongst other disorders.
Nocturnal polysomnography and sleep apnoea
During sleep, episodes may occur in which the flow of air is interrupted or reduced. An apnoea is an interruption in the flow of air that meets specific criteria. A hypopnoea is a partial reduction in breathing that may be associated with a drop in oxygen levels or with waking up.
Polysomnography reveals when these events occur, what happens to respiratory effort, and whether they cause changes in oxygen levels or sleep interruptions. It also reveals whether they are more frequent when the person sleeps on their back or during a particular stage of sleep.
One of its key features is that the equipment records how long the person has actually slept, so that respiratory events can be correlated with the actual duration of sleep.
In selected adults with suspected obstructive apnoea, a home respiratory test may be used. Polysomnography is particularly relevant when a more comprehensive assessment is required or when a home study does not provide sufficient information. Sleep medicine guidelines distinguish between these two approaches and do not regard the home test as a universal substitute for polysomnography.
Where respiratory problems are predominant, Memorial also offers a programme that utilises either home respiratory polygraphy or overnight polysomnography, depending on the clinical complexity.
Differences between polysomnography and home respiratory polygraphy
Both tests can be used in the investigation of nocturnal respiratory disorders, but they do not record exactly the same information.
Polysomnography also analyses signals that indicate whether the person is awake or asleep and which stage of sleep they are in. In addition, it records breathing, oxygen levels, muscle activity and other parameters. Home respiratory polygraphy focuses mainly on breathing, oxygen levels and other associated variables, and is usually carried out at home.
This difference is important because, in a polysomnography, respiratory events can be linked to actual sleep time, whilst certain home-based studies are based on the available recording time.
The choice between the two depends on the clinical question, the patient’s medical history and the complexity of the problem being investigated.
How to prepare for an overnight polysomnography
The unit will provide specific instructions for each test. In general, you should arrive with clean hair and avoid using gels, hairsprays, oils or other products that might make it difficult to attach the electrodes.
It is also usually a good idea to wear comfortable pyjamas and bring any personal items you need for the night. Before the test, it is important to inform the staff about:
Any regular medication.
Any medicines used to help you sleep or stay awake.
Any devices used to treat breathing problems during sleep.
Any previous allergies or skin irritations caused by adhesives.
Any mobility issues or if you need help getting up.
Any relevant needs during the night.
Medicines must not be stopped or their dosing schedules altered on your own initiative.
Guidance regarding caffeine, alcohol, naps or specific treatments depends on the aim of the study and must be followed in accordance with the instructions received.
How a night-time polysomnography is carried out
On arrival at the unit, the staff explain how the test will be carried out and set up the various sensors. Electrodes are placed on the scalp and face, respiratory sensors near the nose and mouth, straps around the chest and abdomen, and a pulse oximeter. Where clinically indicated, sensors may be added to the legs and other monitoring devices may be used.
Fitting the sensors takes some time because, before starting, it is necessary to check that all signals are being received correctly. Afterwards, the patient lies down in the room set aside for the study and tries to sleep as naturally as possible.
The sensors and cables are arranged to allow for changes in position. It is not necessary to remain still throughout the night. Staff monitor the recording from another room and can adjust any sensors if they stop working properly.
If you need to get up to go to the toilet, the team will help manage the connections to make it easier for you to move around.
In certain studies, it may be planned to adjust a respiratory treatment during the night using a device that delivers air pressure. Where this intervention forms part of the test, it will be explained in advance.
What you might feel during the night and what happens at the end
Fitting the sensors does not usually cause any pain, although their presence may feel a little uncomfortable at first. You will feel the straps around your chest and abdomen, and the breathing sensor near your nose may feel a little different to usual.
It is common to take a little longer to fall asleep or to wake up more often because of the environment and the sensors.
It is common for sleep to be somewhat different on the night of the test. The specialist assesses how much sleep has been obtained, which sleep stages have been recorded and the quality of the signals to determine whether the information obtained allows the test to be interpreted correctly. In the morning, the electrodes and sensors are removed. Some gel or adhesive may remain on the skin or hair, but this usually washes off with normal washing. In most cases, no recovery time is needed and you can resume your normal activities, unless otherwise advised by the team. The results are not usually available straight away, as the recording contains many hours of data that must be reviewed and analysed before the report can be drawn up.
What information do the results of a polysomnography provide?
The report does not merely state whether or not a disorder is present. The specialist reconstructs the course of the night and links the various changes recorded.
How sleep was organised
The analysis looks at how long you slept, how long it took you to fall asleep, and how the different sleep stages are distributed.
Wakings are also studied, and sleep efficiency can be calculated, which relates the time actually spent asleep to the time spent in bed.
Breathing, awakenings and oxygen
The study identifies apnoeas, hypopnoeas and other possible respiratory events, and reveals whether they occur particularly during certain phases or in certain positions.
It also shows whether these episodes cause awakenings and what happens to oxygen levels.
The assessment of oxygen levels is not based solely on the lowest recorded value. Duration, frequency and their relationship to respiratory events are also taken into account.
Movements during sleep
The recording can identify repetitive leg movements and assess whether they are associated with awakenings.
When investigating certain nocturnal behaviours, the video and muscle activity allow us to link what is observed to the phase of sleep in which it occurs.
Heart rate
The test records the heart rate and certain changes in heart rhythm throughout the night.
This information forms part of the polysomnography, but does not replace a specific cardiological examination where necessary.
What does the apnoea-hypopnoea index mean?
In sleep apnoea studies, one of the best-known parameters is the apnoea-hypopnoea index (AHI). It expresses the average number of apnoeas and hypopnoeas recorded per hour of sleep.
This is important information, but it does not, on its own, summarise the entire study. The specialist also takes into account symptoms, changes in oxygen levels, awakenings, the sleep stages in which the events occur, body position and other clinical circumstances.
The report may also include parameters relating to sleep continuity, leg movements and other recorded signals.
When the study does not allow a conclusion to be drawn
Sometimes polysomnography does not fully answer the question that prompted the study. This may be because the person slept for only a short time, because a sensor lost its signal during a significant part of the night, or because the episode under investigation did not occur.
There is also natural variation from one night to the next. Even a technically sound recording may not account for all the symptoms.
When this happens, the specialist considers the results alongside the patient’s medical history and may recommend further tests or a repeat polysomnography where necessary. A study showing no significant abnormalities does not rule out all sleep disorders.
Limitations and potential discomforts associated with polysomnography
Polysomnography examines a specific night, and sleeping away from one’s usual environment whilst wearing sensors may alter sleep to some extent. Some disorders occur intermittently and may not manifest during the night under study.
The quality of the results also depends on obtaining sufficient sleep time and on the sensors recording data correctly. Although it provides a wealth of information, it cannot, on its own, explain all the causes of insomnia, tiredness or drowsiness.
The test is non-invasive and does not use ionising radiation. Any discomfort is mainly related to the sensors, straps and adhesive patches and may include mild skin irritation or discomfort during the night.
Polygraphy, actigraphy and other sleep tests
Not all sleep problems require the same test. When the main aim is to investigate possible obstructive sleep apnoea in a specific individual, a home sleep study may be used.
When it is necessary to monitor actual sleep and wakefulness patterns over several days, actigraphy may be used. This technique involves a small, watch-like device that records periods of activity and rest.
Memorial uses polysomnography, actigraphy and other tests depending on the problem being investigated.
Where there is excessive daytime sleepiness, a multiple sleep latency test may be indicated; this is carried out during the day and measures how easily a person falls asleep at various scheduled times. It can form part of the assessment for narcolepsy and other disorders characterised by excessive sleepiness.
Sleep diaries also provide information on sleep schedules, awakenings and perceptions of rest. Each of these tools addresses a different aspect and can complement the others.
For further information on common sleep problems, please refer to the content on Memorial and , which are included amongst the related content for this unit.
Specialists in nocturnal polysomnography at Memorial Publio Cordón
At Memorial Publio Cordón Hospital, nocturnal polysomnography is carried out in the Neurology Department, which forms part of the Brain & Mind Centre. The Neurology Department is headed by Dr Rocío García-Ramos.
The Neurological Sleep Disorders Unit is headed by Dr Alejandro Melcón Villalibre, a specialist in sleep disorders in adults and children, obstructive and central sleep apnoea, and nocturnal polysomnography.
The process begins with a clinical assessment of the sleep problem. When polysomnography is indicated, specially trained staff prepare the sensors and monitor the quality of the recording. The specialist then combines the various signals with the patient’s symptoms and medical history to interpret the study.
Where respiratory disturbances or other problems requiring further assessment are predominant, care can be co-ordinated with specialist units in Pulmonology, Otorhinolaryngology or other areas. The Neurological Sleep Disorders Unit itself facilitates this multidisciplinary co-ordination.
Frequently asked questions about overnight polysomnography
Is polysomnography the same as a sleep study?
‘Sleep study’ is a broad term that can refer to various tests. Nocturnal polysomnography is a test that simultaneously records sleep and various physiological signals throughout the night.
Do you have to sleep through the night for the test to be valid?
Not necessarily. The specialist assesses the recorded sleep duration, the sleep stages observed and the quality of the signals to determine whether there is sufficient information.
Can I move whilst I'm asleep?
Yes. The sensors and cables are positioned to allow for normal changes in position during the night.
What happens if I need to get up to go to the toilet?
The staff supervising the study can help arrange transport to make it easier for participants to get to the study.
Do I need to stop taking any medication before the polysomnography?
Not on your own initiative. Some treatments may affect your sleep, so you must inform the unit and follow their instructions.
Is polysomnography used solely to diagnose sleep apnoea?
No. It can also be used to investigate certain movements, parasomnias, excessive sleepiness and other disorders when it is necessary to analyse what happens during sleep.
How does it differ from a respiratory polygraph test?
Polysomnography records signals that indicate when a person is actually asleep and which stage of sleep they are in. Polysomnography focuses mainly on breathing and oxygen levels.
What is the apnoea-hypopnoea index?
This is the average number of apnoeas and hypopnoeas recorded per hour of sleep. It is an important parameter, but its significance must be interpreted in conjunction with the rest of the study.
Does a normal test rule out any sleep disorder?
No. Some problems may be intermittent or may not occur on that particular night. The result is always based on the symptoms and the rest of the assessment.
When will the result be announced?
It is not usually available by the end of the night. The recorded signals must be checked and analysed before the report can be drawn up.
Medical information for general guidance
This factsheet aims to help you understand what a nocturnal polysomnography is, how it is carried out, what signals it records and what information it can provide in the study of sleep disorders.
Memorial Publio Cordón Hospital promotes accurate, understandable and useful medical information, linked to specialist care and advances in medicine.
This information is for educational purposes only and is not a substitute for the assessment of a healthcare professional or the instructions provided by the Neurological Sleep Disorders Unit. The indication for the test, preparation, the sensors used and the interpretation of the results must be tailored to the clinical situation and the specific objective of the study.