Being told you need a sleep study is a strange moment. You’ve spent months feeling exhausted, your partner has been counting the pauses in your breathing, and the answer to all of it apparently lies in wearing some equipment overnight and waiting for a report. It’s reasonable to want to know what that equipment is actually measuring, and how a night’s recording turns into a diagnosis.

The honest answer is that a sleep study diagnosis obstructive sleep apnoea by recording what your breathing does while you sleep airflow through your nose and mouth, the effort your chest and abdomen make, your blood oxygen levels, and usually your snoring, heart rate and body position then counting how many times per hour your breathing stops or drops significantly. At the Snoring & Sleep Clinic, sleep studies can be performed at home or in selected clinic and hospital settings, with the results used to assess severity, oxygen levels, positional events and treatment options and everything that follows depends on getting that recording right.

What Is a Sleep Study?

A sleep study is an overnight recording of your breathing and, in some cases, your sleep itself. Rather than asking you how well you slept, it measures what happened.

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There are broadly two families of tests. Respiratory polygraphy often done at home records breathing, oxygen and related signals. Polysomnography adds brain wave, eye movement and muscle activity recordings, which means it can identify when you’re actually asleep, which stage of sleep you’re in, and when your sleep is being fragmented.

NICE recommends home respiratory polygraphy as the first-line diagnostic test for suspected obstructive sleep apnoea/hypopnoea syndrome in adults, with more detailed testing reserved for specific circumstances. For most people, that means the diagnosis is made from a night in their own bed rather than a night in hospital.

What Does a Sleep Study Actually Measure?

This is where the mechanics become clearer, because each sensor answers a different question.

Airflow

A small nasal cannula measures pressure changes as you breathe, and sometimes a thermistor detects temperature change at the nose and mouth. This is the primary signal: it shows whether air is actually moving, and how much.

Respiratory Effort

Elastic bands around your chest and abdomen detect the movement of breathing. This channel answers a crucial question that airflow alone cannot: when airflow stops, is your body still trying to breathe? If effort continues while airflow ceases, the airway is obstructed. If effort stops too, the signal to breathe itself has paused a different condition entirely.

Oxygen Levels

A pulse oximeter on your finger records blood oxygen saturation continuously. Obstructive events typically produce a characteristic dip-and-recovery pattern in oxygen, and the depth and frequency of those dips form an important part of the picture, arguably the part with the clearest link to cardiovascular risk.

Snoring, Body Position and Heart Rate

A microphone or vibration sensor records snoring. A position sensor records whether you were on your back, your side or your front. Heart rate comes from the oximeter. These aren’t incidental extras: position data in particular can completely change what treatment makes sense.

What Only a Full Polysomnogram Adds

Polysomnography adds EEG electrodes to record brain activity, EOG for eye movements, EMG for chin and leg muscle activity, and ECG. Together these allow the study to determine:

Exactly when you fell asleep and how long you actually slept
Which sleep stages you spent time in, including REM
Arousals brief awakenings you won’t remember, which fragment sleep even without a big oxygen dip
Leg movements, which point toward other sleep disorders
Whether events cluster in REM sleep specifically

What Counts as an Apnoea or a Hypopnoea?

The definitions are precise, which is what makes the result objective.

An apnoea is a drop in airflow of at least 90% from baseline, lasting at least 10 seconds. In other words, breathing effectively stops.

A hypopnoea is a drop in airflow of at least 30% from baseline, lasting at least 10 seconds, accompanied by either an oxygen desaturation commonly 3% or 4% depending on the scoring criteria in use or an arousal from sleep.

Two things follow from those definitions. First, hypopnoeas count. People often assume only complete stops matter, but partial obstruction that disrupts oxygen or sleep is part of the same disease. Second, the exact scoring criteria a laboratory uses can shift your number somewhat, which is one reason results are interpreted by a clinician rather than read as an absolute.

What Is the AHI, and How Is Severity Graded?

The apnoea–hypopnoea index is the total number of apnoeas and hypopnoeas divided by the hours of sleep or, on home studies without EEG, by hours of recording time. It’s the headline figure on your report.

AHI (events per hour)	Classification
Fewer than 5	Within normal limits
5 to 14.9	Mild obstructive sleep apnoea
15 to 29.9	Moderate obstructive sleep apnoea
30 or more	Severe obstructive sleep apnoea

An important caveat: the AHI alone doesn’t determine whether you need treatment. NICE recommends CPAP alongside lifestyle advice for adults with symptomatic moderate or severe OSAHS and symptoms matter as much as the index. Someone with an AHI of 12 who falls asleep at traffic lights may need treatment more urgently than someone with an AHI of 20 who feels entirely well.

What Do the Other Numbers on Your Report Mean?

The AHI gets the attention, but the rest of the report often carries more clinically useful detail.

Oxygen desaturation index (ODI) how many times per hour your oxygen dropped by a set amount. A useful cross-check on the AHI.
Minimum oxygen saturation, the lowest level reached overnight.
Time below 90% saturates how long you spent with low oxygen in total, which can matter more than how many separate events occurred.
Supine AHI versus non-supine AHI your event rate on your back compared with on your side. A large difference points to positional OSA.
Snoring percentage: how much of the night involved snoring.
Total sleep time or recording time is an important context for the AHI itself.

Which Type of Sleep Study Will You Have?

Study type	What it records	Typically used when
Home respiratory polygraphy	Airflow, respiratory effort, oxygen, snoring, position, heart rate	First-line test for suspected OSA in most adults
Home oximetry	Oxygen saturation and heart rate only	Where polygraphy isn’t available; avoided in heart or lung disease
Clinic or hospital respiratory polygraphy	Same channels as home polygraphy, in a supervised setting	Where home testing isn’t practical or additional monitoring is needed
Polysomnography	Polygraphy plus EEG, EOG, EMG and ECG	Inconclusive or negative polygraphy despite symptoms; another sleep disorder suspected

Why Home Respiratory Polygraphy Is Usually First

Because it works. It measures the signals that define obstructive sleep apnoea, it does so while you sleep in your own bed with your usual pillow and routine, and it avoids the artificial sleep of an unfamiliar hospital room. It’s also quicker to arrange and considerably less costly, which matters when the goal is diagnosing a common condition promptly.

When Is Oximetry Alone Used?

Oximetry records oxygen and heart rate and nothing else. It’s used where fuller testing isn’t available, but it has a real limitation: it can’t tell you why oxygen dropped. In someone with lung or heart disease, desaturations may have nothing to do with airway obstruction, which is why oximetry alone is generally avoided in those patients.

When Is In-Lab Polysomnography Needed?

Polysomnography comes into its own when the simpler test doesn’t settle the question. Common reasons include:

A negative or borderline polygraphy result in someone with convincing symptoms
Suspicion of another sleep disorder narcolepsy, periodic limb movement disorder, parasomnias
Suspected central sleep apnoea or a complex breathing pattern
Significant heart or lung disease complicating interpretation
Assessment of sleep fragmentation where arousals rather than desaturations appear to be the problem

Can a Sleep Study Tell Obstructive From Central Sleep Apnoea?

Yes, and this is where the respiratory effort bands earn their place.

In obstructive apnoea, the brain sends the signal to breathe and the muscles try the chest and abdomen keep moving but the upper airway is blocked, so no air moves. In central apnoea, the drive to breathe itself pauses, so airflow and effort stop together.

Distinguishing them changes everything about management, which is one reason oximetry alone is a limited test: a desaturation looks much the same on an oximeter whichever mechanism caused it.

Can a Sleep Study Miss Sleep Apnoea?

It can, and knowing how is genuinely useful if your result comes back normal but you still feel awful.

Poor sleep at night. If you barely slept, there was little opportunity for events to occur.
Recording time versus sleep time. Home studies without EEG divide events by hours recorded, not hours slept. If you lay awake for two hours, your AHI is diluted.
Sleeping in an unusual position. Someone with positional OSA who happens to spend the night on their side may record a near-normal result.
Sensor problems. A nasal cannula that comes loose, or an oximeter probe that shifts, loses data.
Night-to-night variation. Alcohol, nasal congestion, medication and fatigue all shift results.
REM-predominant OSA. Less REM than usual on the study night means fewer events captured.
Arousal-driven disease. Where sleep fragmentation rather than desaturation is the main problem, a study without EEG can underestimate the impact.

If symptoms are convincing and the first study is negative, that’s precisely the situation NICE identifies for progressing to fuller testing rather than concluding the matter is closed.

Do Smartwatches and Apps Diagnose Sleep Apnoea?

No. Consumer wearables and phone apps can be genuinely useful for flagging a possible problem, and some now include features that screen for breathing disturbances which may be exactly what prompts someone to seek help.

But they don’t measure the channels that define the diagnosis. Without airflow and respiratory effort, they cannot distinguish an apnoea from a hypopnoea, obstructive from central events, or a genuine desaturation from a movement artefact. Treat a wearable alert as a reason to arrange proper testing, not as a result. Equally, don’t treat a reassuring app as evidence that nothing is wrong if your symptoms say otherwise.

The same applies to questionnaires. Tools such as the Epworth Sleepiness Scale and STOP-Bang are helpful for identifying who should be tested. They don’t diagnose anything on their own.

What Actually Happens Before, During and After the Study?

A typical pathway runs as follows:

Consultation your symptoms, sleep history, partner observations, medical history and risk factors are discussed
Examination including assessment of your nose, mouth, palate and throat
Study arranged at home or in a clinic or hospital setting, depending on what’s appropriate
Equipment fitted or explained sensors applied, or shown to you with instructions for home use
Overnight recording you sleep as normally as you can manage, in your usual position and routine
Equipment returned and data downloaded
Scoring and interpretation events counted against defined criteria and the report reviewed by a clinician
Results consultation the findings explained alongside your symptoms, and options discussed

A few practical points make the recording more useful: avoid alcohol and sedatives that night, keep to your usual bedtime, don’t deliberately sleep in an unusual position to be comfortable with the equipment, and don’t worry excessively about sleeping “properly” most people sleep less well than usual and the study still yields a valid result.

Is DISE the Same as a Sleep Study?

No, and the distinction matters because the two are often confused.

A sleep study answers whether you have obstructive sleep apnoea and how severe it is. Drug-Induced Sleep Endoscopy answers a different question: where your airway is collapsing and how. It lets a specialist observe the upper airway under sleep-like conditions and identify whether the obstruction is at the palate, the pharyngeal walls, the tongue base or the epiglottis, and whether the collapse pattern is front-to-back, side-to-side or concentric.

That information isn’t needed to make a diagnosis but it’s central to planning ENT surgery for sleep apnoea or assessing suitability for hypoglossal nerve stimulation. The sequence is diagnosed first, then anatomical assessment if a targeted intervention is being considered.

What Happens After Diagnosis?

The sleep study result shapes what comes next, but it doesn’t dictate it on its own.

NICE recommends fixed-level CPAP alongside lifestyle advice for adults with symptomatic moderate or severe OSAHS, and recognises customised or semi-custom mandibular advancement splints as an option for appropriately selected adults who cannot tolerate or decline CPAP. Where the study shows strongly positional disease, positional measures may form part of the plan. Where CPAP proves intolerable and anatomy is contributing, sleep apnoea treatment in London may extend to DISE-guided surgery or hypoglossal nerve stimulation for carefully selected patients.

NICE also notes that where OSAHS may have been resolved for example after significant weight loss treatment should be reviewed and a repeat sleep assessment considered, rather than assuming the condition has simply gone. Sleep studies aren’t only diagnostic; they’re also how change is measured.

How Are Sleep Studies Arranged in London?

The Snoring & Sleep Clinic’s pathway combines consultation, airway examination and sleep testing, with DISE brought in where more anatomical information is needed before a surgical or implant decision. Sleep studies can be performed at home or in selected clinic and hospital settings, with results used to assess severity, oxygen levels, positional events and treatment options.

The clinic currently offers consultations at Welbeck and at Cleveland Clinic London, the latter at 24 Portland Place, London W1B 1LU. It is led by Mr Ryan Chin Taw Cheong, a Consultant ENT and Sleep Surgeon specialising in snoring and sleep apnoea, who holds a substantive position at the Royal National ENT and Eastman Dental Hospitals (UCLH) and works across both public and private practice.

For anyone searching for a sleep study in the UK, the useful question isn’t which clinic offers the most sophisticated equipment. It’s whether the test chosen is the right one for your symptoms, and whether someone will sit down and explain what the numbers actually mean for you.

About the Specialist Behind This Guide

This guide is written and medically reviewed by Mr Ryan Chin Taw Cheong, who leads the Snoring & Sleep Clinic. He is a Consultant ENT and Sleep Surgeon specialising in snoring and sleep apnoea, and he holds a substantive position at the Royal National ENT and Eastman Dental Hospitals (UCLH), working across both public and private practice. That day-to-day clinical experience reviewing sleep studies, performing Drug-Induced Sleep Endoscopy, and assessing patients for mandibular advancement devices, upper-airway surgery and hypoglossal nerve stimulation is what shapes the emphasis throughout this article on interpreting the whole report rather than a single index, rather than a generic overview of testing written without hands-on specialist experience.

Frequently Asked Questions

How does a sleep study diagnose sleep apnoea?

It records airflow, respiratory effort, oxygen saturation and usually snoring, heart rate and body position overnight, then counts how many times per hour breathing stops or drops significantly. That figure is the apnoea–hypopnoea index.

What is AHI?

The apnoea–hypopnoea index: the average number of apnoeas and hypopnoeas per hour of sleep, or per hour of recording on home studies. It’s the main measure of severity.

What AHI means I have sleep apnoea?

An AHI of 5 or more indicates OSA. Mild is 5 to 14.9, moderate 15 to 29.9, and severe 30 or above. Symptoms are considered alongside the number.

Can a sleep study be done at home?

Yes. NICE recommends home respiratory polygraphy as the first-line test for suspected OSAHS in most adults, and the Snoring & Sleep Clinic performs studies at home as well as in selected clinic and hospital settings.

What’s the difference between a home study and polysomnography?

Home respiratory polygraphy records breathing, oxygen and related signals. Polysomnography adds EEG, eye movement and muscle recordings, so it can measure actual sleep time, sleep stages and arousals.

What’s the difference between an apnoea and a hypopnoea?

An apnoea is a drop in airflow of at least 90% for at least 10 seconds. A hypopnoea is a drop of at least 30% for at least 10 seconds with an associated oxygen desaturation or arousal.

Can a sleep study tell obstructiveness from central sleep apnoea?

Yes, via the respiratory effort bands. In obstructive events effort continues while airflow stops; in central events both stop together.

Can a sleep study miss sleep apnoea?

It can. Poor sleep at night, sleeping in an unusual position, sensor problems, night-to-night variation and REM-predominant disease can all produce a falsely reassuring result. Fuller testing is appropriate where symptoms remain convincing.

Can a smartwatch diagnose sleep apnoea?

No. Wearables can flag a possible problem and prompt testing, but they don’t measure airflow or respiratory effort, so they can’t make the diagnosis or distinguish event types.

Do I need to sleep well for the study to work?

Not perfectly. Most people sleep less well than usual with equipment on and still produce a valid recording. Keeping to your usual bedtime and sleeping position helps.

Is DISE a type of sleep study?

No. A sleep study establishes whether you have OSA and how severe it is. DISE identifies where and how the airway collapses, and is used when surgery or an implant is being considered.

Will I need a repeat sleep study?

Possibly. NICE notes that where OSAHS may have been resolved for example after significant weight loss treatment should be reviewed and repeat assessment considered rather than assumed.

Finding the Right Sleep Apnoea Diagnosis in London

A sleep study turns a set of frustrating symptoms into measurable information: how often your breathing is interrupted, how far your oxygen falls, whether position makes a difference, and whether the events are obstructive or central. That information is what makes a treatment plan possible and it’s also why the report deserves more attention than the single number at the top of it.

If your partner has noticed you stop breathing, if you snore heavily, or if you’re persistently exhausted despite spending enough time in bed, get in touch with the Snoring & Sleep Clinic. Consultations with Mr Ryan Chin Taw Cheong are available at Welbeck and at Cleveland Clinic London, 24 Portland Place, London W1B 1LU, and a specialist assessment there can establish which sleep study is appropriate for you, interpret what it shows, and explain what it means for your options whether that turns out to be CPAP, a mandibular advancement device, positional therapy, surgery guided by DISE, or hypoglossal nerve stimulation.