AHI (Apnea Hypopnea Index)

The AHI refers to the total number of apnea and hypopnea events that occur each hour of sleep.


A complete pause (or cessation) of breathing that lasts at least 10 seconds.


A 30% or greater reduction in breathing that lasts at least 10 seconds and leads to either a) an arousal (micro awakening) from sleep and/or b) a 3% or greater oxygen desaturation.

Collectively, apnea and hypopnea disrupt sleep by causing arousals and drops in blood oxygen saturation. The severity of sleep apnea is measured by the frequency of apnea plus hypopnea each hour of sleep.

0-4 apnea + hypopnea events/hour = normal

5-14 apnea + hypopnea events/hour = mild sleep apnea

15-29 apnea + hypopnea events/hour = moderate sleep apnea

30 or greater apnea + hypopnea events/hour = severe sleep apnea


An arousal is a shift in EEG from sleep to wake that lasts at least 3 seconds. apnea and hypopnea events often cause arousals from sleep because a person will wake briefly to breathe normally again. Most arousals from sleep are brief enough that a person does not remember them. Nonetheless, they are disruptive to sleep and may cause daytime sleepiness or fatigue.


Teeth grinding. This is identified by submental chin EMG activity during a sleep study. Significant bruxism may cause arousals from sleep and treatment with a dental guard may be indicated.

Central Apnea

Cessation of breathing for at least 10 seconds where there is no effort to breathe, as identified by respiratory effort belts. Central apnea is caused by imbalanced respiratory control centres in the brain and is prevalent those with heart failure and history of stroke.

Obesity Hypoventilation

Condition in which severely overweight people fail to breathe rapidly enough or deeply enough, resulting in low blood oxygen levels and high blood carbon dioxide (CO2) levels.

Mixed apnea

A cessation of breathing that lasts at least 10 seconds that is a combination of obstructive and central sleep apnea.


The lowest point. Nadir SaO2 in a sleep study is the lowest oxygen saturation during the night.

Periodic Limb Movement Index (PLMI)

Periodic Limb Movements are leg jerks that occur during sleep, which are typically 5-90 seconds apart. If the leg jerks are robust, they may cause arousals from sleep. The PLMI is a measure of the number of limb movements that cause arousals each hour. When the limb movements cause frequent arousals, treatment with mineral supplements or medication may be indicated.

Primary Snoring

Snoring during sleep with no evidence of sleep disordered breathing or blood gas abnormality.

REM Latency

REM latency refers to the amount of time it takes a person to achieve REM (dream) sleep after the person has initially fallen to sleep. A person normally enters their first period of REM sleep about 45-120 minutes after falling to sleep.

Respiratory Disturbance Index (RDI)

The RDI refers to the total number of apnea events plus hypopnea events plus Respiratory Event Related Arousals (RERAs) that occur each hour of sleep. A RERA is a sequence of breaths characterised by increasing respiratory effort that results in an arousal from sleep.

Sleep Efficiency

Sleep efficiency refers to the percentage of time a person sleeps, in relation to the amount of time a person spends in bed. The percentage is calculated by dividing Total Sleep Time by Total Time in Bed. Normal sleep efficiency is considered to be 80% or greater.

Sleep Latency

Sleep latency refers to the amount of time it takes a person to fall to sleep. Normal sleep latency is 5-15 minutes. Sleep latency less than five minutes may suggest some degree of excessive sleepiness. Sleep latency greater than 20-30 minutes may suggest some difficulty with sleep initiation.

Sleep Architecture

Sleep architecture refers to the structure of sleep. There are four stages of sleep that make up each sleep cycle. Each sleep cycle lasts about 90-120 minutes; thus, a person should have 4-5 cycles of sleep each night.

Stages of Sleep

There are four stages of sleep with the following characteristics:

  • Stage 1 Sleep is the first and earliest stage of sleep. Often referred to as "drowsiness" or "pre-sleep", it is very light sleep. During this stage, individuals usually have an awareness of their surroundings and may not perceive themselves to be asleep. Only about 5-10% of total sleep time is spent in Stage 1 sleep.
  • Stage 2 Sleep is the predominant sleep stage during a normal night of sleep. An individual does not have an awareness of surroundings during this stage and is approaching a deeper state of sleep. About 40-55% of sleep time is spent in this stage.
  • Stage 3 Sleep is deep sleep. During this stage, individuals are more difficult to wake from sleep and if they do wake from this stage, they typically feel drowsy. About 15-25% of sleep is comprised of deep sleep. Older individuals tend to have a lower percentage of deep sleep.
  • Stage 4 Sleep is REM sleep. Rapid Eye Movement (REM) sleep is otherwise referred to as dream sleep. About 10-25% of sleep time is spent in this stage, with the largest majority of REM occurring late in the night.