Glossary: Understanding a Sleep Study

Part of understanding a sleep study is learning some definitions and acronyms used in sleep medicine to describe the many different aspects of how we sleep.

AASM

American Academy of Sleep Medicine

Apnea/ Hypopnea Index (AHI)

The number of apneas and hypopneas that occur per hour of sleep (averaged out over an entire night of sleep).

Arousal

An interruption of sleep which may occur for a variety of reasons (respiratory event, limb movement, snoring, loud noise, no apparent reason, etc). Arousals last 15 seconds or less. If greater than 15 seconds, the epoch is scored as wake.

Bilevel

Positive airway pressure machine that provides dual pressures called ipap/epap (inspiratory pressure/ expiratory pressure). It is better tolerated as pressure required is over 15cm h20. Patients with a history of copd and chf do better with bilevel.

Central Apnea

A period of 10 seconds or more, when there is no airflow measured at the nose and mouth, and there are no attempts to breathe detected (a few of these occurring when first falling asleep can be normal).

sleep apnea testing

CPAP

Continuous positive airway pressure. This is not supplemental oxygen. It is normal, pressurized air that acts as a pneumatic stent or splint to keep the tissues in the airway from collapsing. Normal pressures required are between 5–20cm H20. The protocol at Northshore Sleep Medicine is to consider bilevel when CPAP pressures are 15cm or greater as most patients have a difficult time expiring against such high pressures.

Epoch

A window of time set for viewing events (e.g. sleep staging is done using a 30-second window; respiratory events are viewed with a two-minute window).

Heart Rate

The number of times the heart beats per minute (normal range = 60-100)

Hypopnea

A period of 10 seconds or more when reduced airflow is measured at the nose and mouth, while attempts to breathe continue.

Hypnogram

A graph showing sequentially which stages of sleep a patient entered and for how long.

Mixed Apnea

A combination (or mix), of a central apnea that turns into an obstructive apnea.

Obstructive Apnea

A period of 10 seconds or more, when there is no airflow measured at the nose and mouth, but attempts to breathe continue (breathing attempts are detected by the belts around the chest and abdomen).

Periodic Limb Movement Index (PLMS)

The number of leg movements that occur per hour of sleep (averaged out over an entire night of sleep). Usually only PLMS with arousals are considered significant.

Periodic Limb Movement Index with Arousal

The number of leg movements that occur per hour of sleep which, cause disruption to the sleep (averaged out over an entire night of sleep).

REM Latency

The amount of time it takes to start dreaming from the time one falls asleep. Normal REM latency is about 90 minutes but varies with age.

REM Sleep

Stands for rapid eye movement sleep (vivid dreaming often occurs in rem). Normal amount is approximately 20% and has very little variation over adult life span.

RERA

respiratory effort related arousal

Respiratory Disturbance Index (RDI)

The number of apneas, hypopneas and RERAs that occur per hour of sleep (averaged out over an entire night of sleep).

Respiratory Event

One of five breathing disturbances that may occur during sleep including central apnea, obstructive apnea, mixed apnea, hypopnea and RERA.

Sa02

This abbreviation stands for blood oxygen saturation and is expressed in percent. Normal sa02 = 90% or higher. 

Sleep Efficiency

The amount of time during a sleep study that is actually spent asleep (expressed as a percentage of total time in bed).

Sleep Latency

The amount of time it takes to fall asleep. Normal is up to 30 minutes.

Stage 1 Sleep

The lightest sleep, which normally occurs for a few minutes when first falling asleep. Stage 1 is often seen after an arousal occurs. Normal amount is approximately 6% but varies with age. New AASM guidelines call this N1.

Stage 2 Sleep

A deeper sleep than stage 1. Normal amount is approximately 40–60% but varies with age. New AASM guidelines call this N2.

Stage 3 Sleep

The heaviest or deepest sleep aka slow wave sleep. Normal amounts of this sleep can include from 0-26% but decrease with age. New AASM guidelines call this N3.

Learn more about What to Expect at Your Sleep Visit.

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