Arousal Index Calculator
Arousal Index Calculator
Arousals per hour of sleep, split into the respiratory-related and the rest — because the two sub-indices sum to the total, and which half dominates is the part a single arousal index cannot say.
Arousal index and its two halves
Arousals per hour of sleep214 arousals, of which 131 respiratory-related, over 357 minutes of sleep
Formula
- an arousal
- the criteria the NHLBI National Sleep Research Resource’s reading centre applied, quoting the American Sleep Disorders Association’s 1992 Atlas Task Force: an abrupt shift in EEG frequency, which may include alpha, theta or delta activity or frequencies above 16 Hz, lasting at least 3 seconds and starting after at least 10 continuous seconds of sleep
- the 10-second rule
- it is the half that gets forgotten. Two disturbances five seconds apart cannot both be scored, because the second does not follow ten continuous seconds of sleep. A very fragmented stretch therefore scores fewer arousals than it contains events, and the index understates the fragmentation precisely where the fragmentation is worst
- the denominator
- hours of SCORED SLEEP, not time in bed and not recording time. A home sleep apnoea test cannot produce this index at all: with no EEG it has neither a numerator nor a denominator
- the additive split
- the respiratory and non-respiratory sub-indices sum exactly to the total, because every scored arousal is in one group or the other. That is an identity rather than an approximation, and it is what makes the share below the headline trustworthy
- version 3 added a rule
- the AASM’s public Summary of Updates for version 3 of the scoring manual records a new recommended rule about scoring arousals immediately preceding a transition to stage W. An arousal count is therefore scoring-manual-version dependent as well as scorer dependent
Worked example
214 arousals, of which 131 respiratory-related, over 357 minutes of sleep
Hours of sleep = 357 ÷ 60 = 5.95
Arousal index = 214 ÷ 5.95 = 36.0 arousals/h
Respiratory arousal index = 131 ÷ 5.95 = 22.0/h
From all other causes = (214 − 131) ÷ 5.95 = 83 ÷ 5.95 = 13.9/h
Check the identity: 22.0 + 13.9 = 36.0, the total. Every scored arousal is in one group or the other, so the two sub-indices cannot fail to add up
Respiratory share = 131 ÷ 214 = 61.2%, so most but not all of the fragmentation here is respiratory. Set the respiratory count to 40 instead and the share falls to 18.7%: the same total arousal index, and a night whose fragmentation is mostly not about the breathing
One arousal every 357 ÷ 214 = 1.67 minutes of sleep, which is the figure that makes an index of 36 concrete
Halve the sleep time to 178.5 minutes with the same counts and the index doubles to 71.9. Doubling both counts and the sleep time leaves it at 36.0 — it is a rate, and both checks fail for an implementation that divided by minutes instead of hours
Enter a respiratory count above the total and the page refuses; so does a total sleep time of 0
What the two halves of the index point at
| Sub-index | What raises it | What it is not |
|---|---|---|
| Respiratory arousal index | apnoeas, hypopnoeas and respiratory effort-related arousals ending in a cortical arousal | not the same as the AHI: an event with a desaturation and no arousal counts in the AHI and not here, and a RERA counts here and never in the AHI |
| Arousal index from other causes | periodic limb movements, pain, reflux, environmental noise, the first-night effect, and spontaneous arousals | not a residual category to be ignored: if it dominates, the fragmentation is not being driven by the breathing |
Why no normal range is printed here
| Source | What it gives | Why it is not banded |
|---|---|---|
| OHRI and Sunnybrook normal-PSG calculator | an arousal-index mean with a prediction interval for a given sex, age and night number | it reads a published table, and this engine cannot interpolate one |
| One hospital’s normative sheet | spontaneous arousals above 10 mildly increased, above 20 moderately, above 30 severely | no source is cited for any of the three on the sheet itself, and they are stated for SPONTANEOUS arousals rather than for the total index |
| Your own report | the laboratory’s reference range beside the measurement | nothing — this is the one to use |
The index that explains why two identical AHIs feel different
A respiratory event matters to a patient in two ways: it drops the oxygen and it breaks the sleep. The apnoea-hypopnoea index counts the events and says nothing about either consequence. The arousal index is the measure of the second one — EEG arousals per hour of scored sleep — and it is a large part of why two people with the same AHI can present completely differently.
The split is where the information is. Every scored arousal is either respiratory-related or it is not, so the two sub-indices sum exactly to the total, and the respiratory share is an identity rather than an estimate. A patient with an arousal index of 36 and a respiratory share of 61% has a breathing problem fragmenting their sleep. A patient with the same index of 36 and a respiratory share of 19% has something else doing it — periodic limb movements, pain, reflux, a noisy environment, the first night in an unfamiliar bed — and treating the breathing will not fix the night. A report that prints only the total index cannot distinguish them, which is why the breakdown is the second row here.
The definition is narrower than most readers assume, and it understates fragmentation in exactly the wrong place. The criteria the NHLBI reading centre applied, from the ASDA 1992 Atlas Task Force, require an abrupt EEG frequency shift lasting at least three seconds, and require it to begin after at least ten continuous seconds of sleep. Two disturbances five seconds apart therefore yield one scored arousal, not two. The more broken the sleep, the more the rule bites — so the index is least sensitive where the problem is worst.
It is also manual-version dependent. The AASM’s own Summary of Updates for version 3 records a new recommended rule about scoring arousals immediately preceding a transition to wake, so a count from 2024 and a count from 2015 are not strictly the same measurement. Read this against the reference range your own laboratory printed beside the metric. Normal values here are age- and sex-specific distributions published as tables, and this page does not approximate one. An index is only as meaningful as its denominator: the same event count over a shorter sleep time is a larger index, and nothing about the patient changed. Every number on a sleep study is scoring-rule-dependent and night-to-night variable: the same recording scored under two rules, or the same patient studied on two nights, gives different figures. This page does the arithmetic and says where every figure it quotes comes from. It renders no diagnosis and no clinical decision.
Frequently asked questions
How is the arousal index calculated?
Arousals divided by hours of scored sleep. On the worked night, 214 arousals over 357 minutes is 214 divided by 5.95 hours, which is 36.0 arousals per hour — about one every 1.67 minutes. The denominator is sleep time, not time in bed and not recording time.
What counts as an arousal?
The criteria the NHLBI National Sleep Research Resource’s reading centre used, quoting the ASDA 1992 Atlas Task Force, are an abrupt shift in EEG frequency — alpha, theta, delta or above 16 Hz — lasting at least 3 seconds, starting after at least 10 continuous seconds of sleep. The second condition is the one people forget: two disturbances five seconds apart score as one arousal, so a very broken stretch of sleep produces fewer scored arousals than it contains events.
Why split the index into respiratory and other causes?
Because the two sub-indices sum exactly to the total, so the split tells you what is fragmenting the night. An index of 36 with a 61% respiratory share and an index of 36 with a 19% respiratory share are different clinical situations with the same headline number. A report giving only the total cannot distinguish them.
What arousal index is normal?
It depends on age, which is why no band is printed here. The arousal index rises with age in healthy people, and the reference calculator built on the published meta-analysis needs sex, age and night number before it returns a mean. One hospital sheet read while preparing this page calls spontaneous arousals above 10 per hour mildly increased, above 20 moderately and above 30 severely, cites no source, and states them for spontaneous arousals rather than the total. Use the range your own laboratory printed.
Can a home sleep apnoea test report an arousal index?
No. An arousal is defined on the EEG, and a home respiratory device has no EEG — it can neither detect an arousal nor measure the sleep time to divide by. That is the same gap that stops a home study scoring respiratory effort-related arousals, and the reason its RDI is not the AASM’s RDI.
Related calculators
References
- National Sleep Research Resource (NHLBI). Cleveland Family Study polysomnography reading centre manual: EEG arousal, quoting the American Sleep Disorders Association Atlas Task Force criteria of 1992.
- American Academy of Sleep Medicine. Summary of Updates in Version 3 of The AASM Manual for the Scoring of Sleep and Associated Events. Darien, IL: AASM; February 2023.
- Ottawa Hospital Research Institute / Sunnybrook. Normal Polysomnography Parameters in Healthy Adults — a sex-, age- and night-specific reference calculator built on a published meta-analysis.
- Trillium Health Partners. Normative Values for the Sleep Study Report — cited as an exhibit of one laboratory’s own reference sheet, not as a source of thresholds.
- Kapur VK, Auckley DH, Chowdhuri S, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(3):479–504.
- Mansour M, Won C. Apnea-hypopnea index: limitations and future directions. Pulmonary Health Hub commentary (read as a secondary review; its primary figures were re-read in the papers it cites).
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/