Sleep Study Common Terms
AHI (Apnoea Hypopnoea Index) – The AHI refers to the total number of apnoea and hypopnoea events that occur each hour of sleep.
An apnoea is a complete pause (or cessation) of breathing that lasts at least 10 seconds.
A hypopnoea is a significant reduction in breathing that lasts at least 10 seconds.
Collectively, apnoea and hypopnea disrupt sleep by causing mini awakenings and drops in oxygen levels. The severity of sleep apnoea is measured by the frequency of apnoea and hypopnea each hour.
- 0-5 apnoea + hypopnoea events per hour = normal
- 6-15 apnoea + hypopnoea events per hour = mild sleep apnoea
- 16-29 apnoea + hypopnoea events per hour = moderate sleep apnoea
- 30 or greater apnoea + hypopnoea events per hour = severe sleep apnoea
Arousal – An arousal is a brief awakening from sleep that lasts at least 3 seconds. Apnoea and hypopnoea 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.
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. For example, if a person spends 8 hours in bed (from 10 p.m. to 6 a.m), at least 6.4 hours or more should be spent sleeping to achieve an 80% or greater sleep efficiency. Most healthy and young adults have sleep efficiencies above 90%.
Sleep Latency – Sleep latency refers to the amount of time it takes a person to fall to sleep. For example, if a person goes to bed at 10:00 and falls to sleep at 10:15, the sleep latency is 15 minutes. Normal sleep latency is 5-15 minutes. Sleep latency less than five minutes may suggest some degree of excessive sleepiness. Sleep latency greater than 15 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 – As stated above, there are four stages of sleep with the following descriptions:
- 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 25-45% 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 20-25% of sleep is comprised of deep sleep. Older individuals tend to have a lower percentage of deep sleep.
- Stage R sleep is REM sleep. Rapid Eye Movement (REM) sleep is otherwise referred to as dream sleep. About 20-30% of sleep time is spent in this stage, with the largest majority of REM occurring late in the night.
REM Latency – REM latency refers to the amount of time it takes a person to achieve REM (or dream) sleep after the person has initially fallen to sleep. As REM is the fifth and final stage of a sleep cycle, normal individuals achieve their first period of REM sleep about 70-110 minutes after falling to sleep.
Oxygen Saturation (SaO2) – Oxygen saturation refers to oxygen levels in the blood. Normal oxygen saturation in a healthy adult is 94-100%. In persons with sleep apnoea or lung disease, oxygen saturation levels are often reduced.
Periodic Limb Movement Index (PLMI) – Periodic Limb Movements are leg jerks during sleep that occur every 5-20 seconds. If the leg jerks are robust, they may cause a person to briefly awaken from sleep. The (PLMI) is a measure of the number of leg jerks each hour of sleep. If leg movements are excessive, treatment with supplements or medication may be indicated.