What Happens in a Sleep Study

An overnight recording of brain activity, breathing and movement.

A sleep study, or polysomnography (PSG), is an overnight recording made in a sleep lab of brain waves, eye and muscle activity, breathing, blood oxygen and heart rhythm.¹ It is the reference test for sleep apnea and the first step in diagnosing narcolepsy and idiopathic hypersomnia.^2,3 Sensors are attached in the evening, a technologist monitors from another room, and most people sleep well enough for a useful recording.^1,4 A sleep physician then reads the results alongside your symptoms.⁵

What It Records

Brain waves, eye movements and chin muscle tone. Small electrodes on the scalp, beside the eyes and on the chin show when you are awake, in light or deep sleep, or in REM sleep.¹

Breathing. Sensors at the nose and mouth measure airflow, and soft belts around the chest and abdomen measure breathing effort. Together they show whether a pause in breathing is caused by a blocked airway or by the brain not signaling a breath.⁵

Oxygen and heart rhythm. A clip on a fingertip tracks blood oxygen, and chest leads record heart rhythm.¹

Movement and position. Leg sensors pick up limb movements, and video records body position and any unusual behavior during the night.¹

Why It Is Done

Many adults with suspected sleep apnea can be diagnosed with a simpler home sleep apnea test. An in-lab study is the guideline-recommended choice when a home test has come back negative, inconclusive or technically inadequate, and for people with significant heart or lung disease, neuromuscular weakness, suspected hypoventilation, chronic opioid use, a history of stroke or severe insomnia.² It is also required before the daytime nap test used to diagnose narcolepsy and idiopathic hypersomnia.³

In the Lab or at Home

A home test records breathing and oxygen but not brain activity, so it cannot tell sleep from wakefulness, detect events that only cause a brief arousal, or look for sleep disorders other than apnea.² An in-lab study captures all of that, with a technologist on hand to fix a loose sensor during the night. The tradeoff is cost and a night away from your own bed.

What the Night Is Like

You arrive in the evening and are shown to a private room. A technologist attaches the sensors, which takes a while but is painless, then leaves you to sleep while watching the recording and video from nearby.¹

Most people expect to sleep badly with wires attached. The evidence is more reassuring. In a study of 124 healthy adults who slept in a lab on two consecutive nights, total sleep and sleep efficiency on the first night were not significantly different from the second. The main change was a little more light sleep.⁴

Split-Night Studies

If moderate-to-severe sleep apnea is clear within the first couple of hours, the technologist may spend the rest of the night fitting a CPAP mask and finding the pressure that keeps your airway open. The guideline supports this when at least two hours of diagnostic recording show the problem and at least three hours remain.²

When a Nap Test Follows

When narcolepsy or idiopathic hypersomnia is suspected, the overnight study is followed by a multiple sleep latency test the next day. Some sensors stay on, and you are offered five chances to nap at intervals throughout the day. The test measures how quickly you fall asleep and whether REM sleep appears early, which is the pattern that distinguishes these disorders.³ The overnight study beforehand must show enough sleep, at least six hours, for the nap results to count.³

How to Prepare

Follow the lab’s instructions. In general, avoid caffeine, alcohol and sleeping medications before the study unless told otherwise.¹

Keep your usual medications unless told otherwise. Do not stop a prescribed medication on your own.

If a nap test is planned. A sleep diary or activity monitor for the two weeks before, and any medication changes made under a clinician’s direction, help make the next-day results reliable.³

Reading the Results

A sleep physician scores the recording and writes a report. The headline number for breathing is the apnea-hypopnea index, the count of pauses and shallow-breathing episodes per hour of sleep, explained in understanding AHI and REI.⁵ Reports also cover oxygen levels, time in each sleep stage, how much of the night was spent asleep, and movements during sleep.¹

The numbers are read with your symptoms, not instead of them. The AASM’s testing guideline calls for a sleep study to be done alongside a full sleep evaluation and follow-up.² A study can also miss a problem on a single night. If the first study is negative but suspicion remains, the guideline suggests a second one.²

Taking the Next Step

If loud snoring, pauses in breathing or daytime sleepiness have you wondering about testing, an evaluation can help decide whether a home or in-lab study fits.