A sleep study is any test that records what your body does while you sleep, or how quickly you fall asleep by day. There are four main kinds. A home test tracks breathing and oxygen, and it is mainly used for suspected obstructive sleep apnoea. Full polysomnography (PSG) is an overnight study, usually in a sleep unit, that adds brain waves, eye movements and muscle activity. The multiple sleep latency test (MSLT) is a daytime nap test, done after a PSG, for suspected narcolepsy. The maintenance of wakefulness test (MWT) measures how well you can stay awake.
Which one you are offered depends on the question your doctor is asking. This guide covers what each study records, who usually needs which, and what the results can and cannot tell you. If your question is specifically about a home apnoea test, its price and its weak spots, we cover that in detail separately.
The key difference is brain activity. NICE, the guideline body for England, says PSG adds "brain activity, eye movement and muscle tone signals" to the breathing channels. Those signals measure sleep quality and duration. Without them, a test cannot tell whether you were asleep or which sleep stage you were in.
The US National Library of Medicine's patient guide gives a typical picture. You arrive at the sleep unit about two hours before bedtime. A technician attaches small electrodes to your scalp, chin and the outer edge of your eyelids. Sensors on your chest record breathing and heart rate. Other sensors track movement, and a video camera may record you. A technician watches the recordings through the night.
The study is usually timed to your normal sleep period. Many units can test shift workers during their usual sleep hours. Arrival times and set-up vary between hospitals, so follow your own unit's letter.
The same guide says to avoid sleep medicine, alcohol and caffeinated drinks before the test, unless your sleep specialist says otherwise. Do not stop any prescribed medicine on your own. Ask the sleep unit first.
For most adults with suspected obstructive sleep apnoea, a home test now comes first. NICE recommends offering home respiratory polygraphy. If that is hard to access, it says to consider home oximetry. The American Academy of Sleep Medicine (AASM) accepts either PSG or a technically adequate home test. That applies to uncomplicated adults with signs of moderate to severe sleep apnoea.
Both bodies move to PSG when a home test is not enough:
Questionnaires do not replace a test. The AASM recommends against diagnosing sleep apnoea from questionnaires or prediction tools alone. A screener such as STOP-BANG helps decide who to test. It does not give an answer.
Perhaps your main problem is falling asleep by day despite enough sleep at night. Then the usual route is an overnight PSG, followed by an MSLT the next day. AASM guidance published in 2021 sets out how it should run:
The report gives your average time to fall asleep across the naps, and how many naps included REM sleep. How those numbers feed into a narcolepsy diagnosis, and why it often takes years, is covered in our narcolepsy guide.
The MWT flips the question. Instead of asking you to sleep, it asks you to stay awake. The AASM protocol uses four 40-minute sessions, two hours apart. You sit in a bed or reclining chair in a dimly lit, quiet room. A session ends at the first clear sign of sleep, or after 40 minutes.
The AASM describes two main uses. One is checking how well treatment controls sleepiness. The other is assessing alertness in people who must stay awake for safety reasons. A PSG the night before is optional and left to the clinician.
Many parasomnias do not need a sleep study. AASM practice parameters from 2005 say PSG is not routinely needed for typical, uncomplicated and non-injurious parasomnias when the diagnosis is clear. They say PSG is routinely indicated for sleep behaviours that are violent or potentially injurious, and for atypical or unusual parasomnias.
REM sleep behaviour disorder is the clearest example. In the AASM's 2023 guideline, isolated RBD means dream enactment with a PSG-documented rise in muscle tone during REM sleep. That muscle signal is only visible on a full PSG. Our guide to parasomnias explains how the different types differ.
These two get confused. The NHS describes restless legs syndrome as a strong urge to move your legs, usually when resting at night. The 2005 AASM parameters say PSG is not routinely indicated to diagnose or treat it. Periodic limb movement disorder involves moving your legs often during sleep, and it is one of the conditions PSG can diagnose. The same parameters say PSG may be indicated when there is a strong clinical suspicion of it.
So for restless legs, the first step is usually a conversation and iron tests, not a night in the lab. How to tell restless legs from insomnia is covered separately.
A sleep study is not a routine test for insomnia. AASM practice parameters from 2003 say insomnia is diagnosed mainly from a detailed history. They advise PSG when sleep apnoea or periodic limb movement disorder is suspected, or the diagnosis is uncertain. Treatment failure and violent or injurious arousals are also reasons. The first-line treatment is usually CBT for insomnia, not a test.
Public sleep studies run through hospital sleep services. For example, the HSE lists overnight sleep studies for adults at University Hospital Limerick, with referral from a GP or consultant. Services, waiting times and which tests are offered vary by hospital. Private sleep services also exist. If you have health insurance, ask your insurer what it covers before you book.
The NHS says a GP may refer you to a specialist sleep clinic. Tests can usually be done at home, though sometimes you stay overnight. NICE asks sleep services to prioritise rapid assessment for certain groups. These include people who drive for a living, people whose jobs depend on vigilance for safety, and people who are pregnant.
Testing usually follows a referral or order from a clinician. Cover and costs depend on your insurer. Our home testing guide sets out typical US self-pay ranges and what to check in a quote.
If you are falling asleep at the wheel, stop driving and speak to your doctor now. In the UK, the NHS says that once sleep apnoea with excessive sleepiness is confirmed, you must not drive until symptoms are under control. You may also need to tell the DVLA. In Ireland, ask your GP or sleep specialist how the RSA's medical fitness to drive guidelines apply to you.
No. A home sleep apnoea test records breathing, oxygen and related signals. Full PSG adds brain waves, eye movements and muscle activity. That lets it measure actual sleep and sleep stages. Some guides call home tests "home polysomnography", so check exactly which test you are being offered.
Many people worry about this. The PSG records whatever sleep you get. If the night felt very unusual, tell the technician in the morning.
Do not stop or change anything on your own. Ask the sleep unit well in advance. For an MSLT, AASM guidance says alerting, sedating or REM-affecting medicines should generally be stopped at least two weeks before, under your clinician's plan.
It usually takes most of a day, after the overnight PSG. AASM guidance sets five naps at two-hour intervals, starting 1.5 to 3 hours after you wake.
Usually not. AASM practice parameters say PSG is not routinely needed to diagnose restless legs. A sleep study may be considered if periodic limb movement disorder is strongly suspected.
This article is general information, not medical advice. It cannot diagnose a sleep disorder or tell you which test you need. Talk to your GP or sleep specialist about your own situation.
Reviewed by: Proco Team
Last updated: November 2026