A Plain-English Glossary of 30 Sleep Tracking Terms

Why This Glossary Exists

Read your first sleep study report and it looks like a telemetry dump. AHI. SOREMP. WASO. Sleep efficiency 84.3%. REM latency 112 minutes. Cool, but what does any of it mean?

The sleep tech industry isn't helping. Every wearable invents its own jargon, every app shows a "score" without explaining what's underneath, and the medical literature uses terms unchanged since the 1960s.

This is a working reference. 30+ terms, normal ranges where they exist, citations for every clinical threshold. I leaned on the AASM Manual for the Scoring of Sleep and Associated Events and the open peer-reviewed literature.

None of these definitions diagnose anything. If a number from your tracker (or sleep study) is bothering you, talk to a board-certified sleep specialist. Terms are grouped: stages and architecture, measurement basics, clinical indices, behavioral concepts, tracking metrics, and tests.

Quick reference for the most-asked terms:

Sleep latency: how long it takes to fall asleep. Healthy range is 10 to 20 minutes.

Total Sleep Time (TST): time actually asleep, not time in bed. Adults typically need 7 to 9 hours.

Sleep Efficiency (SE): TST divided by Time in Bed, expressed as a percentage. Normal is 85% or higher.

AHI (Apnea-Hypopnea Index): breathing pauses per hour of sleep. Under 5 is normal in adults; 30+ is severe.

REM: the dreaming stage. Adults spend roughly 20 to 25% of sleep in REM.

Sleep Stages and Architecture

Sleep Architecture

The structure of a night's sleep: the order, duration, and proportion of each stage. A typical adult cycles through NREM and REM 3 to 5 times per night, each cycle lasting 90 to 120 minutes. Deep sleep dominates the first half, REM the second. Source: NCBI Sleep Physiology.

NREM Sleep (N1, N2, N3)

Non-rapid eye movement sleep, divided into three stages. N1 is the brief drowsy transition, usually 5 to 10% of total sleep. N2 is the largest chunk, about 50 to 60%, and includes sleep spindles and K-complexes on EEG. N3 is deep slow-wave sleep, roughly 15 to 20%, where the body does most of its physical recovery. Source: AASM Scoring Manual v3; Carskadon and Dement, Sleep Physiology, NCBI.

REM Sleep

Rapid eye movement sleep. Brain activity looks almost awake, the eyes dart, and most skeletal muscles are temporarily paralyzed (REM atonia). This is when most vivid dreaming happens. Adults spend approximately 20 to 25% of total sleep in REM, with REM periods getting longer toward morning. Source: AASM Scoring Manual v3.

Slow Wave Sleep (SWS) / Deep Sleep

Stage N3, named for the high-amplitude delta waves on EEG. SWS is when growth hormone peaks, glymphatic clearance ramps up, and memory consolidation does heavy lifting. Adults get most of their SWS in the first one or two cycles of the night. Roughly 15 to 20% of total sleep time in healthy adults, and it declines with age. Source: Carskadon and Dement, NCBI Sleep Physiology.

Sleep Cycle

One full progression through NREM stages and into REM, after which the cycle restarts. Adults run 3 to 5 cycles per night, each lasting 90 to 120 minutes. Early cycles are deep-sleep heavy. Later cycles are REM-heavy. Source: NCBI Sleep Physiology.

Sleep Stage

A discrete classification of brain and body activity during sleep, scored in 30-second epochs by polysomnography. The current AASM system uses Wake, N1, N2, N3, and REM (R). The older Rechtschaffen and Kales system split deep sleep into stages 3 and 4. Source: AASM Scoring Manual v3.

Hypnogram

A line graph that plots sleep stages on the y-axis against time on the x-axis. It's the visual signature of one night of sleep. Healthy hypnograms show clean cycling between NREM and REM, deep sleep front-loaded, and few wake intrusions. Source: Wikipedia, Hypnogram; Stephansen et al., JCSM, Characterizing Sleep Structure Using the Hypnogram.

Measurement Basics

Sleep Latency (Sleep Onset Latency)

How long it takes to transition from being awake in bed to the first epoch of sleep. The healthy range is roughly 10 to 20 minutes. Falling asleep in under 5 minutes is often a sign of sleep deprivation, while consistently taking more than 30 minutes meets one criterion for insomnia. Source: AASM Scoring Manual v3. See also our deep dive on sleep latency.

Sleep Onset

The moment you cross from wake into sleep. By AASM rules, sleep onset is scored at the start of the first epoch of any sleep stage (usually N1). It's a marker, not a process. The process is sleep latency. Source: AASM Scoring Manual v3.

Total Sleep Time (TST)

The total minutes spent asleep across all stages during the recording period. TST excludes time awake in bed. Adults generally need 7 to 9 hours per night for healthy function, per the National Sleep Foundation consensus. Source: Hirshkowitz et al., Sleep Health, 2015.

Time in Bed (TIB)

The total time from "lights out" to "lights on," whether you were asleep or not. TIB is the denominator for sleep efficiency. If you're in bed for 8 hours but only sleep for 6, TIB is 8 hours. Source: AASM Scoring Manual v3.

Sleep Efficiency (SE)

TST divided by TIB, multiplied by 100. A simple ratio of how much of your time in bed was actually spent sleeping. Healthy adults score 85% or higher. Below 80% suggests fragmented or insufficient sleep. Source: Ohayon et al., Sleep Health, National Sleep Foundation Quality Recommendations.

Wake After Sleep Onset (WASO)

The total minutes of wakefulness that occur between the first sleep onset and the final morning waking. Includes every middle-of-the-night arousal long enough to be scored as wake. Healthy adults typically have less than 20 minutes of WASO; values above 30 to 40 minutes suggest fragmentation. Source: Ohayon et al., Sleep Health, 2017.

Sleep Fragmentation

The pattern of frequent brief awakenings or arousals that break up sleep continuity, even if you don't remember them. Quantified by the arousal index, WASO, and the number of stage shifts per hour. Heavy fragmentation can leave you feeling unrested despite a full night in bed. Source: AASM Scoring Manual v3.

Polysomnography (PSG)

The full overnight sleep study performed in a lab. PSG records EEG (brain), EOG (eye movements), EMG (muscle tone), ECG (heart), airflow, respiratory effort, SpO2, and body position. It's the gold standard for diagnosing sleep disorders and the reference every consumer tracker is validated against. Source: AASM Scoring Manual v3; Mayo Clinic, Polysomnography.

Actigraphy

A method that estimates sleep and wake from limb movement, usually via an accelerometer on the wrist. Algorithms infer sleep when movement is low and wake when it's high. Studies show roughly 90% sensitivity for sleep but lower specificity for wake. The AASM endorses actigraphy for assessing sleep patterns over multiple days, not as a substitute for PSG. Source: Smith et al., AASM Clinical Practice Guideline, JCSM 2018.

Clinical Indices

These have specific clinical thresholds. They're for context, not self-diagnosis.

Apnea-Hypopnea Index (AHI)

The number of apneas (full breathing pauses) plus hypopneas (partial reductions in airflow with desaturation or arousal) per hour of sleep. Clinical thresholds suggest under 5 is normal in adults, 5 to 14 is mild, 15 to 29 is moderate, and 30 or more is severe sleep apnea. Pediatric thresholds are stricter: an AHI of 1 or higher can indicate OSA in children. Source: AASM Scoring Manual v3; Kapur et al., AASM Diagnostic Testing for OSA, JCSM 2017.

Oxygen Desaturation Index (ODI)

The number of times per hour blood oxygen drops by a defined percentage (typically 3% or 4%) from baseline. The AASM defines ODI on PSG as oxygen desaturations multiplied by 60, divided by total sleep time. ODI tracks closely with AHI and is often used as a screen. Source: AASM Scoring Manual v3; Temirbekov et al., JCSM, 2018.

Arousal Index

The number of EEG arousals per hour of sleep. An arousal is an abrupt shift in brain activity lasting at least 3 seconds (with at least 10 seconds of preceding stable sleep). Arousal index rises with age, by roughly 2 events per hour per decade. Clinical thresholds suggest values above 20 to 30 per hour can indicate significant fragmentation. Source: AASM Scoring Manual v3; Bonnet and Arand, JCSM EEG Arousal Norms by Age.

Periodic Limb Movement (PLM) Index

The number of periodic limb movements per hour of sleep, scored from anterior tibialis EMG. The current ICSD-3 clinical threshold suggests a PLMI greater than 15 per hour in adults (greater than 5 in children) when paired with clinical symptoms. The older threshold of 5 was raised because healthy people often have higher counts. Source: AASM Scoring Manual v3; ICSD-3, American Academy of Sleep Medicine; Ferri et al., Sleep Medicine, 2017.

Sleep Onset REM Period (SOREMP)

A REM period that starts within 15 minutes of sleep onset, instead of the typical 70 to 110 minutes. Two or more SOREMPs across an MSLT, combined with a mean sleep latency under 8 minutes, is one of the diagnostic patterns for narcolepsy under ICSD-3. Source: ICSD-3, AASM; Andlauer et al., Sleep, 2013.

Behavioral and Physiological Concepts

Circadian Rhythm

The roughly 24-hour internal clock that governs sleep-wake timing, hormone release, body temperature, and other functions. The suprachiasmatic nucleus in the hypothalamus is the master pacemaker, entrained primarily by light. Core body temperature reaches its lowest point (the nadir) in the late second half of the sleep period. Source: Czeisler and Gooley, Sleep and Circadian Rhythms, NCBI Bookshelf.

Chronotype

A person's natural tendency toward earlier (morning-type, "lark") or later (evening-type, "owl") sleep timing. Chronotype is partly genetic, shifts with age, and is commonly assessed with the Morningness-Eveningness Questionnaire or the Munich ChronoType Questionnaire. Source: Roenneberg et al., Current Biology, 2007.

Sleep Debt

The cumulative deficit between the sleep you need and the sleep you get. Modest debt can be partially recovered with extra sleep on subsequent nights, but chronic debt produces measurable cognitive and metabolic deficits that don't fully reverse with weekend catch-up. Source: Van Dongen et al., Sleep, 2003. See also our piece on weekend sleep debt.

Sleep Inertia

The grogginess and impaired cognition you feel right after waking. It usually clears within 15 to 30 minutes but can linger for an hour or more, especially if you wake from deep sleep or during your circadian low. Reaction time and complex tasks suffer most. Source: Hilditch and McHill, Nature and Science of Sleep, 2019.

Sleep Pressure

The drive to sleep that builds the longer you stay awake, also called Process S in the two-process model of sleep regulation. Adenosine accumulation in the brain is one of the molecular substrates. Pressure dissipates during sleep, especially deep sleep. Source: Borbely et al., Journal of Sleep Research, 2016.

First-Night Effect

The change in sleep architecture seen on the first night of a sleep study because the environment is unfamiliar. Typical patterns include longer sleep onset, less REM, more wake, and lower sleep efficiency. Sleep apnea and PLM scoring are generally not affected. Source: Agnew et al., Psychophysiology, 1966; Ding et al., Sleep Medicine Reviews, 2022.

Tracking Metrics (What Wearables and Sensors Report)

Body Movement Index

A non-standardized metric used by consumer trackers to summarize how much you moved during sleep. Higher values often (not always) correlate with lighter sleep, more arousals, or restless sleep disorders. There is no agreed-upon clinical threshold. Treat it as a relative trend, not a diagnostic number. Source: Smith et al., AASM Practice Guideline, JCSM 2018.

Position Index

A metric (also non-standardized) that summarizes how much time you spent in each sleep position, plus how often you changed positions. Useful for positional sleep apnea, acid reflux, and pressure injury prevention. The closest clinically validated cousin is "supine AHI" versus "non-supine AHI" in PSG reports. Source: Cartwright, Sleep, 1984; Joosten et al., Sleep Medicine Reviews, 2014.

Heart Rate Variability (HRV)

The beat-to-beat variation in time between heartbeats. Higher HRV during sleep generally signals stronger parasympathetic (recovery) activity. HRV peaks in deep sleep and drops in REM. Normal nocturnal HRV varies widely with age and fitness, often 20 to 100+ ms (RMSSD), so personal trends matter more than absolute values. Source: Stein and Pu, Sleep Medicine Reviews, 2012; Burr, Heart Rate Variability During Specific Sleep Stages, Circulation 1995.

Respiratory Rate

Breaths per minute, measured during sleep. Healthy adults run 12 to 20 breaths per minute at rest. Rates tend to be lowest and most regular in deep sleep, more variable in REM. Sustained values under 12 or over 25 during sleep warrant a medical look. Source: American Lung Association, Vital Signs; Sleep Foundation, Sleep Respiratory Rate.

Core Body Temperature

The temperature of the body's deep tissues, around 36.5 to 37.5 C in healthy adults. It follows a circadian cycle, peaking in the late afternoon and reaching its nadir 1 to 3 hours before the habitual wake time. Sleep onset typically occurs during the downslope of core temperature. Source: Kraeuchi et al., American Journal of Physiology, 1998; Harding et al., Frontiers in Neuroscience, 2019.

Sleep Tests

Multiple Sleep Latency Test (MSLT)

A daytime test that measures how quickly someone falls asleep across 4 to 5 nap opportunities, spaced 2 hours apart, after a full night of recorded sleep. Mean sleep latency under 8 minutes is consistent with pathological sleepiness. Two or more SOREMPs supports a narcolepsy diagnosis. Source: Littner et al., AASM Practice Parameter, Sleep 2005; Arand and Bonnet, Sleep Medicine Reviews, 2019.

Maintenance of Wakefulness Test (MWT)

The opposite of the MSLT. The patient sits in a quiet, dimly lit room and tries to stay awake during four 40-minute trials. Used to assess whether treatment is working or whether someone (truck drivers, pilots) can stay alert. Mean sleep latency above 30 minutes is generally considered normal. Source: Littner et al., AASM Practice Parameter, Sleep 2005.

Home Sleep Apnea Test (HSAT)

A simplified, portable sleep study run at home, usually recording airflow, respiratory effort, oximetry, and sometimes heart rate. AASM guidelines support HSATs as an alternative to in-lab PSG for adults with high pretest probability of moderate-to-severe OSA, but not for complex cases. Source: Kapur et al., AASM Diagnostic Testing for OSA, JCSM 2017.

What Komori Is Being Designed to Measure (And What It Won't)

Komori is a pre-launch contactless monitor in development (Kickstarter Q1 2027). It is not yet shipping and is not a medical device. The honest mapping to the terms above reflects design intent, not verified consumer claims:

Komori is being designed to measure (subject to engineering and FDA-dialogue verification):

Body Movement Index (radar-derived movement signal) (planned)
Position Index (sleep position, position changes, time per position) (planned)
Respiratory Rate (chest displacement from radar) (planned, verification in progress)
Bed exit and return (time of departure, duration out) (planned)
Sleep and wake estimates (planned — movement-based heuristic, similar in spirit to actigraphy, not validated against PSG for staging)

Komori is also not designed to do any of the following — and will not claim to:

REM, N1, N2, or N3 staging (no EEG)
Heart Rate Variability (HRV) (radar can pick up cardiac motion, but Komori does not report HRV as a clinical metric)
AHI, ODI, or SpO2 (no airflow sensor, no oximeter)
Arousal index (requires EEG)
Sleep latency to the minute (estimated, not scored against EEG epochs)

If a marketing page tells you a contactless sensor stages REM with the same accuracy as PSG, ask for the validation paper. Most don't have one.

Why position-first? Position data is what most people need for the conditions they live with: positional sleep apnea, acid reflux, pregnancy, post-surgical recovery, chronic pain, fall risk, dementia wandering. See why we don't show a sleep score and what your movement says about sleep.

How to Actually Use This

A glossary won't fix bad sleep, but it will help you read your data without getting bullied by jargon. Three rules:

Trends beat single nights. Illness, alcohol, and a noisy neighbor warp one night. Watch the rolling average.
Architecture beats score. A weekly average of TST, SE, WASO, and position shifts says more than any "sleep score."
If a number scares you, ask a sleep specialist. Trackers flag patterns. Specialists diagnose conditions.

For a closer look at radar-only sleep data, see our research page and product overview. Definitions here are paraphrased from the AASM Manual v3 (copyrighted) and the open peer-reviewed literature. Clinicians should go to the primary sources linked above. Talk to a sleep specialist for anything that feels off. A glossary is a flashlight, not a map.